--- title: "Dockerfile reference" description: "Dockerfiles use a simple DSL which allows you to automate the steps you would normally manually take to create an image." keywords: "builder, docker, Dockerfile, automation, image creation" redirect_from: - /reference/builder/ --- # Dockerfile reference Docker can build images automatically by reading the instructions from a `Dockerfile`. A `Dockerfile` is a text document that contains all the commands a user could call on the command line to assemble an image. Using `docker build` users can create an automated build that executes several command-line instructions in succession. This page describes the commands you can use in a `Dockerfile`. When you are done reading this page, refer to the [`Dockerfile` Best Practices](https://docs.docker.com/engine/userguide/eng-image/dockerfile_best-practices/) for a tip-oriented guide. ## Usage The [`docker build`](commandline/build.md) command builds an image from a `Dockerfile` and a *context*. The build's context is the files at a specified location `PATH` or `URL`. The `PATH` is a directory on your local filesystem. The `URL` is a Git repository location. A context is processed recursively. So, a `PATH` includes any subdirectories and the `URL` includes the repository and its submodules. A simple build command that uses the current directory as context: $ docker build . Sending build context to Docker daemon 6.51 MB ... The build is run by the Docker daemon, not by the CLI. The first thing a build process does is send the entire context (recursively) to the daemon. In most cases, it's best to start with an empty directory as context and keep your Dockerfile in that directory. Add only the files needed for building the Dockerfile. >**Warning**: Do not use your root directory, `/`, as the `PATH` as it causes >the build to transfer the entire contents of your hard drive to the Docker >daemon. To use a file in the build context, the `Dockerfile` refers to the file specified in an instruction, for example, a `COPY` instruction. To increase the build's performance, exclude files and directories by adding a `.dockerignore` file to the context directory. For information about how to [create a `.dockerignore` file](#dockerignore-file) see the documentation on this page. Traditionally, the `Dockerfile` is called `Dockerfile` and located in the root of the context. You use the `-f` flag with `docker build` to point to a Dockerfile anywhere in your file system. $ docker build -f /path/to/a/Dockerfile . You can specify a repository and tag at which to save the new image if the build succeeds: $ docker build -t shykes/myapp . To tag the image into multiple repositories after the build, add multiple `-t` parameters when you run the `build` command: $ docker build -t shykes/myapp:1.0.2 -t shykes/myapp:latest . Before the Docker daemon runs the instructions in the `Dockerfile`, it performs a preliminary validation of the `Dockerfile` and returns an error if the syntax is incorrect: $ docker build -t test/myapp . Sending build context to Docker daemon 2.048 kB Error response from daemon: Unknown instruction: RUNCMD The Docker daemon runs the instructions in the `Dockerfile` one-by-one, committing the result of each instruction to a new image if necessary, before finally outputting the ID of your new image. The Docker daemon will automatically clean up the context you sent. Note that each instruction is run independently, and causes a new image to be created - so `RUN cd /tmp` will not have any effect on the next instructions. Whenever possible, Docker will re-use the intermediate images (cache), to accelerate the `docker build` process significantly. This is indicated by the `Using cache` message in the console output. (For more information, see the [Build cache section](https://docs.docker.com/engine/userguide/eng-image/dockerfile_best-practices/#build-cache) in the `Dockerfile` best practices guide): $ docker build -t svendowideit/ambassador . Sending build context to Docker daemon 15.36 kB Step 1/4 : FROM alpine:3.2 ---> 31f630c65071 Step 2/4 : MAINTAINER SvenDowideit@home.org.au ---> Using cache ---> 2a1c91448f5f Step 3/4 : RUN apk update && apk add socat && rm -r /var/cache/ ---> Using cache ---> 21ed6e7fbb73 Step 4/4 : CMD env | grep _TCP= | (sed 's/.*_PORT_\([0-9]*\)_TCP=tcp:\/\/\(.*\):\(.*\)/socat -t 100000000 TCP4-LISTEN:\1,fork,reuseaddr TCP4:\2:\3 \&/' && echo wait) | sh ---> Using cache ---> 7ea8aef582cc Successfully built 7ea8aef582cc Build cache is only used from images that have a local parent chain. This means that these images were created by previous builds or the whole chain of images was loaded with `docker load`. If you wish to use build cache of a specific image you can specify it with `--cache-from` option. Images specified with `--cache-from` do not need to have a parent chain and may be pulled from other registries. When you're done with your build, you're ready to look into [*Pushing a repository to its registry*](https://docs.docker.com/engine/tutorials/dockerrepos/#/contributing-to-docker-hub). ## Format Here is the format of the `Dockerfile`: ```Dockerfile # Comment INSTRUCTION arguments ``` The instruction is not case-sensitive. However, convention is for them to be UPPERCASE to distinguish them from arguments more easily. Docker runs instructions in a `Dockerfile` in order. A `Dockerfile` **must start with a \`FROM\` instruction**. The `FROM` instruction specifies the [*Base Image*](glossary.md#base-image) from which you are building. `FROM` may only be proceeded by one or more `ARG` instructions, which declare arguments that are used in `FROM` lines in the `Dockerfile`. Docker treats lines that *begin* with `#` as a comment, unless the line is a valid [parser directive](#parser-directives). A `#` marker anywhere else in a line is treated as an argument. This allows statements like: ```Dockerfile # Comment RUN echo 'we are running some # of cool things' ``` Line continuation characters are not supported in comments. ## Parser directives Parser directives are optional, and affect the way in which subsequent lines in a `Dockerfile` are handled. Parser directives do not add layers to the build, and will not be shown as a build step. Parser directives are written as a special type of comment in the form `# directive=value`. A single directive may only be used once. Once a comment, empty line or builder instruction has been processed, Docker no longer looks for parser directives. Instead it treats anything formatted as a parser directive as a comment and does not attempt to validate if it might be a parser directive. Therefore, all parser directives must be at the very top of a `Dockerfile`. Parser directives are not case-sensitive. However, convention is for them to be lowercase. Convention is also to include a blank line following any parser directives. Line continuation characters are not supported in parser directives. Due to these rules, the following examples are all invalid: Invalid due to line continuation: ```Dockerfile # direc \ tive=value ``` Invalid due to appearing twice: ```Dockerfile # directive=value1 # directive=value2 FROM ImageName ``` Treated as a comment due to appearing after a builder instruction: ```Dockerfile FROM ImageName # directive=value ``` Treated as a comment due to appearing after a comment which is not a parser directive: ```Dockerfile # About my dockerfile # directive=value FROM ImageName ``` The unknown directive is treated as a comment due to not being recognized. In addition, the known directive is treated as a comment due to appearing after a comment which is not a parser directive. ```Dockerfile # unknowndirective=value # knowndirective=value ``` Non line-breaking whitespace is permitted in a parser directive. Hence, the following lines are all treated identically: ```Dockerfile #directive=value # directive =value # directive= value # directive = value # dIrEcTiVe=value ``` The following parser directive is supported: * `escape` ## escape # escape=\ (backslash) Or # escape=` (backtick) The `escape` directive sets the character used to escape characters in a `Dockerfile`. If not specified, the default escape character is `\`. The escape character is used both to escape characters in a line, and to escape a newline. This allows a `Dockerfile` instruction to span multiple lines. Note that regardless of whether the `escape` parser directive is included in a `Dockerfile`, *escaping is not performed in a `RUN` command, except at the end of a line.* Setting the escape character to `` ` `` is especially useful on `Windows`, where `\` is the directory path separator. `` ` `` is consistent with [Windows PowerShell](https://technet.microsoft.com/en-us/library/hh847755.aspx). Consider the following example which would fail in a non-obvious way on `Windows`. The second `\` at the end of the second line would be interpreted as an escape for the newline, instead of a target of the escape from the first `\`. Similarly, the `\` at the end of the third line would, assuming it was actually handled as an instruction, cause it be treated as a line continuation. The result of this dockerfile is that second and third lines are considered a single instruction: ```Dockerfile FROM microsoft/nanoserver COPY testfile.txt c:\\ RUN dir c:\ ``` Results in: PS C:\John> docker build -t cmd . Sending build context to Docker daemon 3.072 kB Step 1/2 : FROM microsoft/nanoserver ---> 22738ff49c6d Step 2/2 : COPY testfile.txt c:\RUN dir c: GetFileAttributesEx c:RUN: The system cannot find the file specified. PS C:\John> One solution to the above would be to use `/` as the target of both the `COPY` instruction, and `dir`. However, this syntax is, at best, confusing as it is not natural for paths on `Windows`, and at worst, error prone as not all commands on `Windows` support `/` as the path separator. By adding the `escape` parser directive, the following `Dockerfile` succeeds as expected with the use of natural platform semantics for file paths on `Windows`: # escape=` FROM microsoft/nanoserver COPY testfile.txt c:\ RUN dir c:\ Results in: PS C:\John> docker build -t succeeds --no-cache=true . Sending build context to Docker daemon 3.072 kB Step 1/3 : FROM microsoft/nanoserver ---> 22738ff49c6d Step 2/3 : COPY testfile.txt c:\ ---> 96655de338de Removing intermediate container 4db9acbb1682 Step 3/3 : RUN dir c:\ ---> Running in a2c157f842f5 Volume in drive C has no label. Volume Serial Number is 7E6D-E0F7 Directory of c:\ 10/05/2016 05:04 PM 1,894 License.txt 10/05/2016 02:22 PM Program Files 10/05/2016 02:14 PM Program Files (x86) 10/28/2016 11:18 AM 62 testfile.txt 10/28/2016 11:20 AM Users 10/28/2016 11:20 AM Windows 2 File(s) 1,956 bytes 4 Dir(s) 21,259,096,064 bytes free ---> 01c7f3bef04f Removing intermediate container a2c157f842f5 Successfully built 01c7f3bef04f PS C:\John> ## Environment replacement Environment variables (declared with [the `ENV` statement](#env)) can also be used in certain instructions as variables to be interpreted by the `Dockerfile`. Escapes are also handled for including variable-like syntax into a statement literally. Environment variables are notated in the `Dockerfile` either with `$variable_name` or `${variable_name}`. They are treated equivalently and the brace syntax is typically used to address issues with variable names with no whitespace, like `${foo}_bar`. The `${variable_name}` syntax also supports a few of the standard `bash` modifiers as specified below: * `${variable:-word}` indicates that if `variable` is set then the result will be that value. If `variable` is not set then `word` will be the result. * `${variable:+word}` indicates that if `variable` is set then `word` will be the result, otherwise the result is the empty string. In all cases, `word` can be any string, including additional environment variables. Escaping is possible by adding a `\` before the variable: `\$foo` or `\${foo}`, for example, will translate to `$foo` and `${foo}` literals respectively. Example (parsed representation is displayed after the `#`): FROM busybox ENV foo /bar WORKDIR ${foo} # WORKDIR /bar ADD . $foo # ADD . /bar COPY \$foo /quux # COPY $foo /quux Environment variables are supported by the following list of instructions in the `Dockerfile`: * `ADD` * `COPY` * `ENV` * `EXPOSE` * `FROM` * `LABEL` * `STOPSIGNAL` * `USER` * `VOLUME` * `WORKDIR` as well as: * `ONBUILD` (when combined with one of the supported instructions above) > **Note**: > prior to 1.4, `ONBUILD` instructions did **NOT** support environment > variable, even when combined with any of the instructions listed above. Environment variable substitution will use the same value for each variable throughout the entire instruction. In other words, in this example: ENV abc=hello ENV abc=bye def=$abc ENV ghi=$abc will result in `def` having a value of `hello`, not `bye`. However, `ghi` will have a value of `bye` because it is not part of the same instruction that set `abc` to `bye`. ## .dockerignore file Before the docker CLI sends the context to the docker daemon, it looks for a file named `.dockerignore` in the root directory of the context. If this file exists, the CLI modifies the context to exclude files and directories that match patterns in it. This helps to avoid unnecessarily sending large or sensitive files and directories to the daemon and potentially adding them to images using `ADD` or `COPY`. The CLI interprets the `.dockerignore` file as a newline-separated list of patterns similar to the file globs of Unix shells. For the purposes of matching, the root of the context is considered to be both the working and the root directory. For example, the patterns `/foo/bar` and `foo/bar` both exclude a file or directory named `bar` in the `foo` subdirectory of `PATH` or in the root of the git repository located at `URL`. Neither excludes anything else. If a line in `.dockerignore` file starts with `#` in column 1, then this line is considered as a comment and is ignored before interpreted by the CLI. Here is an example `.dockerignore` file: ``` # comment */temp* */*/temp* temp? ``` This file causes the following build behavior: | Rule | Behavior | |----------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | `# comment` | Ignored. | | `*/temp*` | Exclude files and directories whose names start with `temp` in any immediate subdirectory of the root. For example, the plain file `/somedir/temporary.txt` is excluded, as is the directory `/somedir/temp`. | | `*/*/temp*` | Exclude files and directories starting with `temp` from any subdirectory that is two levels below the root. For example, `/somedir/subdir/temporary.txt` is excluded. | | `temp?` | Exclude files and directories in the root directory whose names are a one-character extension of `temp`. For example, `/tempa` and `/tempb` are excluded. Matching is done using Go's [filepath.Match](http://golang.org/pkg/path/filepath#Match) rules. A preprocessing step removes leading and trailing whitespace and eliminates `.` and `..` elements using Go's [filepath.Clean](http://golang.org/pkg/path/filepath/#Clean). Lines that are blank after preprocessing are ignored. Beyond Go's filepath.Match rules, Docker also supports a special wildcard string `**` that matches any number of directories (including zero). For example, `**/*.go` will exclude all files that end with `.go` that are found in all directories, including the root of the build context. Lines starting with `!` (exclamation mark) can be used to make exceptions to exclusions. The following is an example `.dockerignore` file that uses this mechanism: ``` *.md !README.md ``` All markdown files *except* `README.md` are excluded from the context. The placement of `!` exception rules influences the behavior: the last line of the `.dockerignore` that matches a particular file determines whether it is included or excluded. Consider the following example: ``` *.md !README*.md README-secret.md ``` No markdown files are included in the context except README files other than `README-secret.md`. Now consider this example: ``` *.md README-secret.md !README*.md ``` All of the README files are included. The middle line has no effect because `!README*.md` matches `README-secret.md` and comes last. You can even use the `.dockerignore` file to exclude the `Dockerfile` and `.dockerignore` files. These files are still sent to the daemon because it needs them to do its job. But the `ADD` and `COPY` instructions do not copy them to the image. Finally, you may want to specify which files to include in the context, rather than which to exclude. To achieve this, specify `*` as the first pattern, followed by one or more `!` exception patterns. **Note**: For historical reasons, the pattern `.` is ignored. ## FROM FROM [AS ] Or FROM [:] [AS ] Or FROM [@] [AS ] The `FROM` instruction initializes a new build stage and sets the [*Base Image*](glossary.md#base-image) for subsequent instructions. As such, a valid `Dockerfile` must start with a `FROM` instruction. The image can be any valid image – it is especially easy to start by **pulling an image** from the [*Public Repositories*](https://docs.docker.com/engine/tutorials/dockerrepos/). - `ARG` is the only instruction that may proceed `FROM` in the `Dockerfile`. See [Understand how ARG and FROM interact](#understand-how-arg-and-from-interact). - `FROM` can appear multiple times within a single `Dockerfile` to create multiple images or use one build stage as a dependency for another. Simply make a note of the last image ID output by the commit before each new `FROM` instruction. Each `FROM` instruction clears any state created by previous instructions. - Optionally a name can be given to a new build stage by adding `AS name` to the `FROM` instruction. The name can be used in subsequent `FROM` and `COPY --from=` instructions to refer to the image built in this stage. - The `tag` or `digest` values are optional. If you omit either of them, the builder assumes a `latest` tag by default. The builder returns an error if it cannot find the `tag` value. ### Understand how ARG and FROM interact `FROM` instructions support variables that are declared by any `ARG` instructions that occur before the first `FROM`. ```Dockerfile ARG CODE_VERSION=latest FROM base:${CODE_VERSION} CMD /code/run-app FROM extras:${CODE_VERSION} CMD /code/run-extras ``` To use the default value of an `ARG` declared before the first `FROM` use an `ARG` instruction without a value: ```Dockerfile ARG SETTINGS=default FROM busybox ARG SETTINGS ``` ## RUN RUN has 2 forms: - `RUN ` (*shell* form, the command is run in a shell, which by default is `/bin/sh -c` on Linux or `cmd /S /C` on Windows) - `RUN ["executable", "param1", "param2"]` (*exec* form) The `RUN` instruction will execute any commands in a new layer on top of the current image and commit the results. The resulting committed image will be used for the next step in the `Dockerfile`. Layering `RUN` instructions and generating commits conforms to the core concepts of Docker where commits are cheap and containers can be created from any point in an image's history, much like source control. The *exec* form makes it possible to avoid shell string munging, and to `RUN` commands using a base image that does not contain the specified shell executable. The default shell for the *shell* form can be changed using the `SHELL` command. In the *shell* form you can use a `\` (backslash) to continue a single RUN instruction onto the next line. For example, consider these two lines: ``` RUN /bin/bash -c 'source $HOME/.bashrc; \ echo $HOME' ``` Together they are equivalent to this single line: ``` RUN /bin/bash -c 'source $HOME/.bashrc; echo $HOME' ``` > **Note**: > To use a different shell, other than '/bin/sh', use the *exec* form > passing in the desired shell. For example, > `RUN ["/bin/bash", "-c", "echo hello"]` > **Note**: > The *exec* form is parsed as a JSON array, which means that > you must use double-quotes (") around words not single-quotes ('). > **Note**: > Unlike the *shell* form, the *exec* form does not invoke a command shell. > This means that normal shell processing does not happen. For example, > `RUN [ "echo", "$HOME" ]` will not do variable substitution on `$HOME`. > If you want shell processing then either use the *shell* form or execute > a shell directly, for example: `RUN [ "sh", "-c", "echo $HOME" ]`. > When using the exec form and executing a shell directly, as in the case for > the shell form, it is the shell that is doing the environment variable > expansion, not docker. > > **Note**: > In the *JSON* form, it is necessary to escape backslashes. This is > particularly relevant on Windows where the backslash is the path separator. > The following line would otherwise be treated as *shell* form due to not > being valid JSON, and fail in an unexpected way: > `RUN ["c:\windows\system32\tasklist.exe"]` > The correct syntax for this example is: > `RUN ["c:\\windows\\system32\\tasklist.exe"]` The cache for `RUN` instructions isn't invalidated automatically during the next build. The cache for an instruction like `RUN apt-get dist-upgrade -y` will be reused during the next build. The cache for `RUN` instructions can be invalidated by using the `--no-cache` flag, for example `docker build --no-cache`. See the [`Dockerfile` Best Practices guide](https://docs.docker.com/engine/userguide/eng-image/dockerfile_best-practices/#/build-cache) for more information. The cache for `RUN` instructions can be invalidated by `ADD` instructions. See [below](#add) for details. ### Known issues (RUN) - [Issue 783](https://github.com/docker/docker/issues/783) is about file permissions problems that can occur when using the AUFS file system. You might notice it during an attempt to `rm` a file, for example. For systems that have recent aufs version (i.e., `dirperm1` mount option can be set), docker will attempt to fix the issue automatically by mounting the layers with `dirperm1` option. More details on `dirperm1` option can be found at [`aufs` man page](https://github.com/sfjro/aufs3-linux/tree/aufs3.18/Documentation/filesystems/aufs) If your system doesn't have support for `dirperm1`, the issue describes a workaround. ## CMD The `CMD` instruction has three forms: - `CMD ["executable","param1","param2"]` (*exec* form, this is the preferred form) - `CMD ["param1","param2"]` (as *default parameters to ENTRYPOINT*) - `CMD command param1 param2` (*shell* form) There can only be one `CMD` instruction in a `Dockerfile`. If you list more than one `CMD` then only the last `CMD` will take effect. **The main purpose of a `CMD` is to provide defaults for an executing container.** These defaults can include an executable, or they can omit the executable, in which case you must specify an `ENTRYPOINT` instruction as well. > **Note**: > If `CMD` is used to provide default arguments for the `ENTRYPOINT` > instruction, both the `CMD` and `ENTRYPOINT` instructions should be specified > with the JSON array format. > **Note**: > The *exec* form is parsed as a JSON array, which means that > you must use double-quotes (") around words not single-quotes ('). > **Note**: > Unlike the *shell* form, the *exec* form does not invoke a command shell. > This means that normal shell processing does not happen. For example, > `CMD [ "echo", "$HOME" ]` will not do variable substitution on `$HOME`. > If you want shell processing then either use the *shell* form or execute > a shell directly, for example: `CMD [ "sh", "-c", "echo $HOME" ]`. > When using the exec form and executing a shell directly, as in the case for > the shell form, it is the shell that is doing the environment variable > expansion, not docker. When used in the shell or exec formats, the `CMD` instruction sets the command to be executed when running the image. If you use the *shell* form of the `CMD`, then the `` will execute in `/bin/sh -c`: FROM ubuntu CMD echo "This is a test." | wc - If you want to **run your** `` **without a shell** then you must express the command as a JSON array and give the full path to the executable. **This array form is the preferred format of `CMD`.** Any additional parameters must be individually expressed as strings in the array: FROM ubuntu CMD ["/usr/bin/wc","--help"] If you would like your container to run the same executable every time, then you should consider using `ENTRYPOINT` in combination with `CMD`. See [*ENTRYPOINT*](#entrypoint). If the user specifies arguments to `docker run` then they will override the default specified in `CMD`. > **Note**: > Don't confuse `RUN` with `CMD`. `RUN` actually runs a command and commits > the result; `CMD` does not execute anything at build time, but specifies > the intended command for the image. ## LABEL LABEL = = = ... The `LABEL` instruction adds metadata to an image. A `LABEL` is a key-value pair. To include spaces within a `LABEL` value, use quotes and backslashes as you would in command-line parsing. A few usage examples: LABEL "com.example.vendor"="ACME Incorporated" LABEL com.example.label-with-value="foo" LABEL version="1.0" LABEL description="This text illustrates \ that label-values can span multiple lines." An image can have more than one label. To specify multiple labels, Docker recommends combining labels into a single `LABEL` instruction where possible. Each `LABEL` instruction produces a new layer which can result in an inefficient image if you use many labels. This example results in a single image layer. LABEL multi.label1="value1" multi.label2="value2" other="value3" The above can also be written as: LABEL multi.label1="value1" \ multi.label2="value2" \ other="value3" Labels are additive including `LABEL`s in `FROM` images. If Docker encounters a label/key that already exists, the new value overrides any previous labels with identical keys. To view an image's labels, use the `docker inspect` command. "Labels": { "com.example.vendor": "ACME Incorporated" "com.example.label-with-value": "foo", "version": "1.0", "description": "This text illustrates that label-values can span multiple lines.", "multi.label1": "value1", "multi.label2": "value2", "other": "value3" }, ## MAINTAINER (deprecated) MAINTAINER The `MAINTAINER` instruction sets the *Author* field of the generated images. The `LABEL` instruction is a much more flexible version of this and you should use it instead, as it enables setting any metadata you require, and can be viewed easily, for example with `docker inspect`. To set a label corresponding to the `MAINTAINER` field you could use: LABEL maintainer="SvenDowideit@home.org.au" This will then be visible from `docker inspect` with the other labels. ## EXPOSE EXPOSE [...] The `EXPOSE` instruction informs Docker that the container listens on the specified network ports at runtime. `EXPOSE` does not make the ports of the container accessible to the host. To do that, you must use either the `-p` flag to publish a range of ports or the `-P` flag to publish all of the exposed ports. You can expose one port number and publish it externally under another number. To set up port redirection on the host system, see [using the -P flag](run.md#expose-incoming-ports). The Docker network feature supports creating networks without the need to expose ports within the network, for detailed information see the [overview of this feature](https://docs.docker.com/engine/userguide/networking/)). ## ENV ENV ENV = ... The `ENV` instruction sets the environment variable `` to the value ``. This value will be in the environment of all "descendant" `Dockerfile` commands and can be [replaced inline](#environment-replacement) in many as well. The `ENV` instruction has two forms. The first form, `ENV `, will set a single variable to a value. The entire string after the first space will be treated as the `` - including characters such as spaces and quotes. The second form, `ENV = ...`, allows for multiple variables to be set at one time. Notice that the second form uses the equals sign (=) in the syntax, while the first form does not. Like command line parsing, quotes and backslashes can be used to include spaces within values. For example: ENV myName="John Doe" myDog=Rex\ The\ Dog \ myCat=fluffy and ENV myName John Doe ENV myDog Rex The Dog ENV myCat fluffy will yield the same net results in the final image, but the first form is preferred because it produces a single cache layer. The environment variables set using `ENV` will persist when a container is run from the resulting image. You can view the values using `docker inspect`, and change them using `docker run --env =`. > **Note**: > Environment persistence can cause unexpected side effects. For example, > setting `ENV DEBIAN_FRONTEND noninteractive` may confuse apt-get > users on a Debian-based image. To set a value for a single command, use > `RUN = `. ## ADD ADD has two forms: - `ADD ... ` - `ADD ["",... ""]` (this form is required for paths containing whitespace) The `ADD` instruction copies new files, directories or remote file URLs from `` and adds them to the filesystem of the image at the path ``. Multiple `` resource may be specified but if they are files or directories then they must be relative to the source directory that is being built (the context of the build). Each `` may contain wildcards and matching will be done using Go's [filepath.Match](http://golang.org/pkg/path/filepath#Match) rules. For example: ADD hom* /mydir/ # adds all files starting with "hom" ADD hom?.txt /mydir/ # ? is replaced with any single character, e.g., "home.txt" The `` is an absolute path, or a path relative to `WORKDIR`, into which the source will be copied inside the destination container. ADD test relativeDir/ # adds "test" to `WORKDIR`/relativeDir/ ADD test /absoluteDir/ # adds "test" to /absoluteDir/ When adding files or directories that contain special characters (such as `[` and `]`), you need to escape those paths following the Golang rules to prevent them from being treated as a matching pattern. For example, to add a file named `arr[0].txt`, use the following; ADD arr[[]0].txt /mydir/ # copy a file named "arr[0].txt" to /mydir/ All new files and directories are created with a UID and GID of 0. In the case where `` is a remote file URL, the destination will have permissions of 600. If the remote file being retrieved has an HTTP `Last-Modified` header, the timestamp from that header will be used to set the `mtime` on the destination file. However, like any other file processed during an `ADD`, `mtime` will not be included in the determination of whether or not the file has changed and the cache should be updated. > **Note**: > If you build by passing a `Dockerfile` through STDIN (`docker > build - < somefile`), there is no build context, so the `Dockerfile` > can only contain a URL based `ADD` instruction. You can also pass a > compressed archive through STDIN: (`docker build - < archive.tar.gz`), > the `Dockerfile` at the root of the archive and the rest of the > archive will be used as the context of the build. > **Note**: > If your URL files are protected using authentication, you > will need to use `RUN wget`, `RUN curl` or use another tool from > within the container as the `ADD` instruction does not support > authentication. > **Note**: > The first encountered `ADD` instruction will invalidate the cache for all > following instructions from the Dockerfile if the contents of `` have > changed. This includes invalidating the cache for `RUN` instructions. > See the [`Dockerfile` Best Practices guide](https://docs.docker.com/engine/userguide/eng-image/dockerfile_best-practices/#/build-cache) for more information. `ADD` obeys the following rules: - The `` path must be inside the *context* of the build; you cannot `ADD ../something /something`, because the first step of a `docker build` is to send the context directory (and subdirectories) to the docker daemon. - If `` is a URL and `` does not end with a trailing slash, then a file is downloaded from the URL and copied to ``. - If `` is a URL and `` does end with a trailing slash, then the filename is inferred from the URL and the file is downloaded to `/`. For instance, `ADD http://example.com/foobar /` would create the file `/foobar`. The URL must have a nontrivial path so that an appropriate filename can be discovered in this case (`http://example.com` will not work). - If `` is a directory, the entire contents of the directory are copied, including filesystem metadata. > **Note**: > The directory itself is not copied, just its contents. - If `` is a *local* tar archive in a recognized compression format (identity, gzip, bzip2 or xz) then it is unpacked as a directory. Resources from *remote* URLs are **not** decompressed. When a directory is copied or unpacked, it has the same behavior as `tar -x`, the result is the union of: 1. Whatever existed at the destination path and 2. The contents of the source tree, with conflicts resolved in favor of "2." on a file-by-file basis. > **Note**: > Whether a file is identified as a recognized compression format or not > is done solely based on the contents of the file, not the name of the file. > For example, if an empty file happens to end with `.tar.gz` this will not > be recognized as a compressed file and **will not** generate any kind of > decompression error message, rather the file will simply be copied to the > destination. - If `` is any other kind of file, it is copied individually along with its metadata. In this case, if `` ends with a trailing slash `/`, it will be considered a directory and the contents of `` will be written at `/base()`. - If multiple `` resources are specified, either directly or due to the use of a wildcard, then `` must be a directory, and it must end with a slash `/`. - If `` does not end with a trailing slash, it will be considered a regular file and the contents of `` will be written at ``. - If `` doesn't exist, it is created along with all missing directories in its path. ## COPY COPY has two forms: - `COPY ... ` - `COPY ["",... ""]` (this form is required for paths containing whitespace) The `COPY` instruction copies new files or directories from `` and adds them to the filesystem of the container at the path ``. Multiple `` resource may be specified but they must be relative to the source directory that is being built (the context of the build). Each `` may contain wildcards and matching will be done using Go's [filepath.Match](http://golang.org/pkg/path/filepath#Match) rules. For example: COPY hom* /mydir/ # adds all files starting with "hom" COPY hom?.txt /mydir/ # ? is replaced with any single character, e.g., "home.txt" The `` is an absolute path, or a path relative to `WORKDIR`, into which the source will be copied inside the destination container. COPY test relativeDir/ # adds "test" to `WORKDIR`/relativeDir/ COPY test /absoluteDir/ # adds "test" to /absoluteDir/ When copying files or directories that contain special characters (such as `[` and `]`), you need to escape those paths following the Golang rules to prevent them from being treated as a matching pattern. For example, to copy a file named `arr[0].txt`, use the following; COPY arr[[]0].txt /mydir/ # copy a file named "arr[0].txt" to /mydir/ All new files and directories are created with a UID and GID of 0. > **Note**: > If you build using STDIN (`docker build - < somefile`), there is no > build context, so `COPY` can't be used. Optionally `COPY` accepts a flag `--from=` that can be used to set the source location to a previous build stage (created with `FROM .. AS `) that will be used instead of a build context sent by the user. The flag also accepts a numeric index assigned for all previous build stages started with `FROM` instruction. In case a build stage with a specified name can't be found an image with the same name is attempted to be used instead. `COPY` obeys the following rules: - The `` path must be inside the *context* of the build; you cannot `COPY ../something /something`, because the first step of a `docker build` is to send the context directory (and subdirectories) to the docker daemon. - If `` is a directory, the entire contents of the directory are copied, including filesystem metadata. > **Note**: > The directory itself is not copied, just its contents. - If `` is any other kind of file, it is copied individually along with its metadata. In this case, if `` ends with a trailing slash `/`, it will be considered a directory and the contents of `` will be written at `/base()`. - If multiple `` resources are specified, either directly or due to the use of a wildcard, then `` must be a directory, and it must end with a slash `/`. - If `` does not end with a trailing slash, it will be considered a regular file and the contents of `` will be written at ``. - If `` doesn't exist, it is created along with all missing directories in its path. ## ENTRYPOINT ENTRYPOINT has two forms: - `ENTRYPOINT ["executable", "param1", "param2"]` (*exec* form, preferred) - `ENTRYPOINT command param1 param2` (*shell* form) An `ENTRYPOINT` allows you to configure a container that will run as an executable. For example, the following will start nginx with its default content, listening on port 80: docker run -i -t --rm -p 80:80 nginx Command line arguments to `docker run ` will be appended after all elements in an *exec* form `ENTRYPOINT`, and will override all elements specified using `CMD`. This allows arguments to be passed to the entry point, i.e., `docker run -d` will pass the `-d` argument to the entry point. You can override the `ENTRYPOINT` instruction using the `docker run --entrypoint` flag. The *shell* form prevents any `CMD` or `run` command line arguments from being used, but has the disadvantage that your `ENTRYPOINT` will be started as a subcommand of `/bin/sh -c`, which does not pass signals. This means that the executable will not be the container's `PID 1` - and will _not_ receive Unix signals - so your executable will not receive a `SIGTERM` from `docker stop `. Only the last `ENTRYPOINT` instruction in the `Dockerfile` will have an effect. ### Exec form ENTRYPOINT example You can use the *exec* form of `ENTRYPOINT` to set fairly stable default commands and arguments and then use either form of `CMD` to set additional defaults that are more likely to be changed. FROM ubuntu ENTRYPOINT ["top", "-b"] CMD ["-c"] When you run the container, you can see that `top` is the only process: $ docker run -it --rm --name test top -H top - 08:25:00 up 7:27, 0 users, load average: 0.00, 0.01, 0.05 Threads: 1 total, 1 running, 0 sleeping, 0 stopped, 0 zombie %Cpu(s): 0.1 us, 0.1 sy, 0.0 ni, 99.7 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st KiB Mem: 2056668 total, 1616832 used, 439836 free, 99352 buffers KiB Swap: 1441840 total, 0 used, 1441840 free. 1324440 cached Mem PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND 1 root 20 0 19744 2336 2080 R 0.0 0.1 0:00.04 top To examine the result further, you can use `docker exec`: $ docker exec -it test ps aux USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND root 1 2.6 0.1 19752 2352 ? Ss+ 08:24 0:00 top -b -H root 7 0.0 0.1 15572 2164 ? R+ 08:25 0:00 ps aux And you can gracefully request `top` to shut down using `docker stop test`. The following `Dockerfile` shows using the `ENTRYPOINT` to run Apache in the foreground (i.e., as `PID 1`): ``` FROM debian:stable RUN apt-get update && apt-get install -y --force-yes apache2 EXPOSE 80 443 VOLUME ["/var/www", "/var/log/apache2", "/etc/apache2"] ENTRYPOINT ["/usr/sbin/apache2ctl", "-D", "FOREGROUND"] ``` If you need to write a starter script for a single executable, you can ensure that the final executable receives the Unix signals by using `exec` and `gosu` commands: ```bash #!/usr/bin/env bash set -e if [ "$1" = 'postgres' ]; then chown -R postgres "$PGDATA" if [ -z "$(ls -A "$PGDATA")" ]; then gosu postgres initdb fi exec gosu postgres "$@" fi exec "$@" ``` Lastly, if you need to do some extra cleanup (or communicate with other containers) on shutdown, or are co-ordinating more than one executable, you may need to ensure that the `ENTRYPOINT` script receives the Unix signals, passes them on, and then does some more work: ``` #!/bin/sh # Note: I've written this using sh so it works in the busybox container too # USE the trap if you need to also do manual cleanup after the service is stopped, # or need to start multiple services in the one container trap "echo TRAPed signal" HUP INT QUIT TERM # start service in background here /usr/sbin/apachectl start echo "[hit enter key to exit] or run 'docker stop '" read # stop service and clean up here echo "stopping apache" /usr/sbin/apachectl stop echo "exited $0" ``` If you run this image with `docker run -it --rm -p 80:80 --name test apache`, you can then examine the container's processes with `docker exec`, or `docker top`, and then ask the script to stop Apache: ```bash $ docker exec -it test ps aux USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND root 1 0.1 0.0 4448 692 ? Ss+ 00:42 0:00 /bin/sh /run.sh 123 cmd cmd2 root 19 0.0 0.2 71304 4440 ? Ss 00:42 0:00 /usr/sbin/apache2 -k start www-data 20 0.2 0.2 360468 6004 ? Sl 00:42 0:00 /usr/sbin/apache2 -k start www-data 21 0.2 0.2 360468 6000 ? Sl 00:42 0:00 /usr/sbin/apache2 -k start root 81 0.0 0.1 15572 2140 ? R+ 00:44 0:00 ps aux $ docker top test PID USER COMMAND 10035 root {run.sh} /bin/sh /run.sh 123 cmd cmd2 10054 root /usr/sbin/apache2 -k start 10055 33 /usr/sbin/apache2 -k start 10056 33 /usr/sbin/apache2 -k start $ /usr/bin/time docker stop test test real 0m 0.27s user 0m 0.03s sys 0m 0.03s ``` > **Note:** you can override the `ENTRYPOINT` setting using `--entrypoint`, > but this can only set the binary to *exec* (no `sh -c` will be used). > **Note**: > The *exec* form is parsed as a JSON array, which means that > you must use double-quotes (") around words not single-quotes ('). > **Note**: > Unlike the *shell* form, the *exec* form does not invoke a command shell. > This means that normal shell processing does not happen. For example, > `ENTRYPOINT [ "echo", "$HOME" ]` will not do variable substitution on `$HOME`. > If you want shell processing then either use the *shell* form or execute > a shell directly, for example: `ENTRYPOINT [ "sh", "-c", "echo $HOME" ]`. > When using the exec form and executing a shell directly, as in the case for > the shell form, it is the shell that is doing the environment variable > expansion, not docker. ### Shell form ENTRYPOINT example You can specify a plain string for the `ENTRYPOINT` and it will execute in `/bin/sh -c`. This form will use shell processing to substitute shell environment variables, and will ignore any `CMD` or `docker run` command line arguments. To ensure that `docker stop` will signal any long running `ENTRYPOINT` executable correctly, you need to remember to start it with `exec`: FROM ubuntu ENTRYPOINT exec top -b When you run this image, you'll see the single `PID 1` process: $ docker run -it --rm --name test top Mem: 1704520K used, 352148K free, 0K shrd, 0K buff, 140368121167873K cached CPU: 5% usr 0% sys 0% nic 94% idle 0% io 0% irq 0% sirq Load average: 0.08 0.03 0.05 2/98 6 PID PPID USER STAT VSZ %VSZ %CPU COMMAND 1 0 root R 3164 0% 0% top -b Which will exit cleanly on `docker stop`: $ /usr/bin/time docker stop test test real 0m 0.20s user 0m 0.02s sys 0m 0.04s If you forget to add `exec` to the beginning of your `ENTRYPOINT`: FROM ubuntu ENTRYPOINT top -b CMD --ignored-param1 You can then run it (giving it a name for the next step): $ docker run -it --name test top --ignored-param2 Mem: 1704184K used, 352484K free, 0K shrd, 0K buff, 140621524238337K cached CPU: 9% usr 2% sys 0% nic 88% idle 0% io 0% irq 0% sirq Load average: 0.01 0.02 0.05 2/101 7 PID PPID USER STAT VSZ %VSZ %CPU COMMAND 1 0 root S 3168 0% 0% /bin/sh -c top -b cmd cmd2 7 1 root R 3164 0% 0% top -b You can see from the output of `top` that the specified `ENTRYPOINT` is not `PID 1`. If you then run `docker stop test`, the container will not exit cleanly - the `stop` command will be forced to send a `SIGKILL` after the timeout: $ docker exec -it test ps aux PID USER COMMAND 1 root /bin/sh -c top -b cmd cmd2 7 root top -b 8 root ps aux $ /usr/bin/time docker stop test test real 0m 10.19s user 0m 0.04s sys 0m 0.03s ### Understand how CMD and ENTRYPOINT interact Both `CMD` and `ENTRYPOINT` instructions define what command gets executed when running a container. There are few rules that describe their co-operation. 1. Dockerfile should specify at least one of `CMD` or `ENTRYPOINT` commands. 2. `ENTRYPOINT` should be defined when using the container as an executable. 3. `CMD` should be used as a way of defining default arguments for an `ENTRYPOINT` command or for executing an ad-hoc command in a container. 4. `CMD` will be overridden when running the container with alternative arguments. The table below shows what command is executed for different `ENTRYPOINT` / `CMD` combinations: | | No ENTRYPOINT | ENTRYPOINT exec_entry p1_entry | ENTRYPOINT ["exec_entry", "p1_entry"] | |--------------------------------|----------------------------|--------------------------------|------------------------------------------------| | **No CMD** | *error, not allowed* | /bin/sh -c exec_entry p1_entry | exec_entry p1_entry | | **CMD ["exec_cmd", "p1_cmd"]** | exec_cmd p1_cmd | /bin/sh -c exec_entry p1_entry | exec_entry p1_entry exec_cmd p1_cmd | | **CMD ["p1_cmd", "p2_cmd"]** | p1_cmd p2_cmd | /bin/sh -c exec_entry p1_entry | exec_entry p1_entry p1_cmd p2_cmd | | **CMD exec_cmd p1_cmd** | /bin/sh -c exec_cmd p1_cmd | /bin/sh -c exec_entry p1_entry | exec_entry p1_entry /bin/sh -c exec_cmd p1_cmd | ## VOLUME VOLUME ["/data"] The `VOLUME` instruction creates a mount point with the specified name and marks it as holding externally mounted volumes from native host or other containers. The value can be a JSON array, `VOLUME ["/var/log/"]`, or a plain string with multiple arguments, such as `VOLUME /var/log` or `VOLUME /var/log /var/db`. For more information/examples and mounting instructions via the Docker client, refer to [*Share Directories via Volumes*](https://docs.docker.com/engine/tutorials/dockervolumes/#/mount-a-host-directory-as-a-data-volume) documentation. The `docker run` command initializes the newly created volume with any data that exists at the specified location within the base image. For example, consider the following Dockerfile snippet: FROM ubuntu RUN mkdir /myvol RUN echo "hello world" > /myvol/greeting VOLUME /myvol This Dockerfile results in an image that causes `docker run`, to create a new mount point at `/myvol` and copy the `greeting` file into the newly created volume. ### Notes about specifying volumes Keep the following things in mind about volumes in the `Dockerfile`. - **Volumes on Windows-based containers**: When using Windows-based containers, the destination of a volume inside the container must be one of: - a non-existing or empty directory - a drive other than `C:` - **Changing the volume from within the Dockerfile**: If any build steps change the data within the volume after it has been declared, those changes will be discarded. - **JSON formatting**: The list is parsed as a JSON array. You must enclose words with double quotes (`"`)rather than single quotes (`'`). - **The host directory is declared at container run-time**: The host directory (the mountpoint) is, by its nature, host-dependent. This is to preserve image portability. since a given host directory can't be guaranteed to be available on all hosts.For this reason, you can't mount a host directory from within the Dockerfile. The `VOLUME` instruction does not support specifying a `host-dir` parameter. You must specify the mountpoint when you create or run the container. ## USER USER daemon The `USER` instruction sets the user name or UID to use when running the image and for any `RUN`, `CMD` and `ENTRYPOINT` instructions that follow it in the `Dockerfile`. ## WORKDIR WORKDIR /path/to/workdir The `WORKDIR` instruction sets the working directory for any `RUN`, `CMD`, `ENTRYPOINT`, `COPY` and `ADD` instructions that follow it in the `Dockerfile`. If the `WORKDIR` doesn't exist, it will be created even if it's not used in any subsequent `Dockerfile` instruction. It can be used multiple times in the one `Dockerfile`. If a relative path is provided, it will be relative to the path of the previous `WORKDIR` instruction. For example: WORKDIR /a WORKDIR b WORKDIR c RUN pwd The output of the final `pwd` command in this `Dockerfile` would be `/a/b/c`. The `WORKDIR` instruction can resolve environment variables previously set using `ENV`. You can only use environment variables explicitly set in the `Dockerfile`. For example: ENV DIRPATH /path WORKDIR $DIRPATH/$DIRNAME RUN pwd The output of the final `pwd` command in this `Dockerfile` would be `/path/$DIRNAME` ## ARG ARG [=] The `ARG` instruction defines a variable that users can pass at build-time to the builder with the `docker build` command using the `--build-arg =` flag. If a user specifies a build argument that was not defined in the Dockerfile, the build outputs a warning. ``` [Warning] One or more build-args [foo] were not consumed. ``` The Dockerfile author can define a single variable by specifying `ARG` once or many variables by specifying `ARG` more than once. For example, a valid Dockerfile: ``` FROM busybox ARG user1 ARG buildno ... ``` A Dockerfile author may optionally specify a default value for an `ARG` instruction: ``` FROM busybox ARG user1=someuser ARG buildno=1 ... ``` If an `ARG` value has a default and if there is no value passed at build-time, the builder uses the default. An `ARG` variable definition comes into effect from the line on which it is defined in the `Dockerfile` not from the argument's use on the command-line or elsewhere. For example, consider this Dockerfile: ``` 1 FROM busybox 2 USER ${user:-some_user} 3 ARG user 4 USER $user ... ``` A user builds this file by calling: ``` $ docker build --build-arg user=what_user . ``` The `USER` at line 2 evaluates to `some_user` as the `user` variable is defined on the subsequent line 3. The `USER` at line 4 evaluates to `what_user` as `user` is defined and the `what_user` value was passed on the command line. Prior to its definition by an `ARG` instruction, any use of a variable results in an empty string. > **Warning:** It is not recommended to use build-time variables for > passing secrets like github keys, user credentials etc. Build-time variable > values are visible to any user of the image with the `docker history` command. You can use an `ARG` or an `ENV` instruction to specify variables that are available to the `RUN` instruction. Environment variables defined using the `ENV` instruction always override an `ARG` instruction of the same name. Consider this Dockerfile with an `ENV` and `ARG` instruction. ``` 1 FROM ubuntu 2 ARG CONT_IMG_VER 3 ENV CONT_IMG_VER v1.0.0 4 RUN echo $CONT_IMG_VER ``` Then, assume this image is built with this command: ``` $ docker build --build-arg CONT_IMG_VER=v2.0.1 . ``` In this case, the `RUN` instruction uses `v1.0.0` instead of the `ARG` setting passed by the user:`v2.0.1` This behavior is similar to a shell script where a locally scoped variable overrides the variables passed as arguments or inherited from environment, from its point of definition. Using the example above but a different `ENV` specification you can create more useful interactions between `ARG` and `ENV` instructions: ``` 1 FROM ubuntu 2 ARG CONT_IMG_VER 3 ENV CONT_IMG_VER ${CONT_IMG_VER:-v1.0.0} 4 RUN echo $CONT_IMG_VER ``` Unlike an `ARG` instruction, `ENV` values are always persisted in the built image. Consider a docker build without the `--build-arg` flag: ``` $ docker build . ``` Using this Dockerfile example, `CONT_IMG_VER` is still persisted in the image but its value would be `v1.0.0` as it is the default set in line 3 by the `ENV` instruction. The variable expansion technique in this example allows you to pass arguments from the command line and persist them in the final image by leveraging the `ENV` instruction. Variable expansion is only supported for [a limited set of Dockerfile instructions.](#environment-replacement) Docker has a set of predefined `ARG` variables that you can use without a corresponding `ARG` instruction in the Dockerfile. * `HTTP_PROXY` * `http_proxy` * `HTTPS_PROXY` * `https_proxy` * `FTP_PROXY` * `ftp_proxy` * `NO_PROXY` * `no_proxy` To use these, simply pass them on the command line using the flag: ``` --build-arg = ``` By default, these pre-defined variables are excluded from the output of `docker history`. Excluding them reduces the risk of accidentally leaking sensitive authentication information in an `HTTP_PROXY` variable. For example, consider building the following Dockerfile using `--build-arg HTTP_PROXY=http://user:pass@proxy.lon.example.com` ``` Dockerfile FROM ubuntu RUN echo "Hello World" ``` In this case, the value of the `HTTP_PROXY` variable is not available in the `docker history` and is not cached. If you were to change location, and your proxy server changed to `http://user:pass@proxy.sfo.example.com`, a subsequent build does not result in a cache miss. If you need to override this behaviour then you may do so by adding an `ARG` statement in the Dockerfile as follows: ``` Dockerfile FROM ubuntu ARG HTTP_PROXY RUN echo "Hello World" ``` When building this Dockerfile, the `HTTP_PROXY` is preserved in the `docker history`, and changing its value invalidates the build cache. ### Impact on build caching `ARG` variables are not persisted into the built image as `ENV` variables are. However, `ARG` variables do impact the build cache in similar ways. If a Dockerfile defines an `ARG` variable whose value is different from a previous build, then a "cache miss" occurs upon its first usage, not its definition. In particular, all `RUN` instructions following an `ARG` instruction use the `ARG` variable implicitly (as an environment variable), thus can cause a cache miss. All predefined `ARG` variables are exempt from caching unless there is a matching `ARG` statement in the `Dockerfile`. For example, consider these two Dockerfile: ``` 1 FROM ubuntu 2 ARG CONT_IMG_VER 3 RUN echo $CONT_IMG_VER ``` ``` 1 FROM ubuntu 2 ARG CONT_IMG_VER 3 RUN echo hello ``` If you specify `--build-arg CONT_IMG_VER=` on the command line, in both cases, the specification on line 2 does not cause a cache miss; line 3 does cause a cache miss.`ARG CONT_IMG_VER` causes the RUN line to be identified as the same as running `CONT_IMG_VER=` echo hello, so if the `` changes, we get a cache miss. Consider another example under the same command line: ``` 1 FROM ubuntu 2 ARG CONT_IMG_VER 3 ENV CONT_IMG_VER $CONT_IMG_VER 4 RUN echo $CONT_IMG_VER ``` In this example, the cache miss occurs on line 3. The miss happens because the variable's value in the `ENV` references the `ARG` variable and that variable is changed through the command line. In this example, the `ENV` command causes the image to include the value. If an `ENV` instruction overrides an `ARG` instruction of the same name, like this Dockerfile: ``` 1 FROM ubuntu 2 ARG CONT_IMG_VER 3 ENV CONT_IMG_VER hello 4 RUN echo $CONT_IMG_VER ``` Line 3 does not cause a cache miss because the value of `CONT_IMG_VER` is a constant (`hello`). As a result, the environment variables and values used on the `RUN` (line 4) doesn't change between builds. ## ONBUILD ONBUILD [INSTRUCTION] The `ONBUILD` instruction adds to the image a *trigger* instruction to be executed at a later time, when the image is used as the base for another build. The trigger will be executed in the context of the downstream build, as if it had been inserted immediately after the `FROM` instruction in the downstream `Dockerfile`. Any build instruction can be registered as a trigger. This is useful if you are building an image which will be used as a base to build other images, for example an application build environment or a daemon which may be customized with user-specific configuration. For example, if your image is a reusable Python application builder, it will require application source code to be added in a particular directory, and it might require a build script to be called *after* that. You can't just call `ADD` and `RUN` now, because you don't yet have access to the application source code, and it will be different for each application build. You could simply provide application developers with a boilerplate `Dockerfile` to copy-paste into their application, but that is inefficient, error-prone and difficult to update because it mixes with application-specific code. The solution is to use `ONBUILD` to register advance instructions to run later, during the next build stage. Here's how it works: 1. When it encounters an `ONBUILD` instruction, the builder adds a trigger to the metadata of the image being built. The instruction does not otherwise affect the current build. 2. At the end of the build, a list of all triggers is stored in the image manifest, under the key `OnBuild`. They can be inspected with the `docker inspect` command. 3. Later the image may be used as a base for a new build, using the `FROM` instruction. As part of processing the `FROM` instruction, the downstream builder looks for `ONBUILD` triggers, and executes them in the same order they were registered. If any of the triggers fail, the `FROM` instruction is aborted which in turn causes the build to fail. If all triggers succeed, the `FROM` instruction completes and the build continues as usual. 4. Triggers are cleared from the final image after being executed. In other words they are not inherited by "grand-children" builds. For example you might add something like this: [...] ONBUILD ADD . /app/src ONBUILD RUN /usr/local/bin/python-build --dir /app/src [...] > **Warning**: Chaining `ONBUILD` instructions using `ONBUILD ONBUILD` isn't allowed. > **Warning**: The `ONBUILD` instruction may not trigger `FROM` or `MAINTAINER` instructions. ## STOPSIGNAL STOPSIGNAL signal The `STOPSIGNAL` instruction sets the system call signal that will be sent to the container to exit. This signal can be a valid unsigned number that matches a position in the kernel's syscall table, for instance 9, or a signal name in the format SIGNAME, for instance SIGKILL. ## HEALTHCHECK The `HEALTHCHECK` instruction has two forms: * `HEALTHCHECK [OPTIONS] CMD command` (check container health by running a command inside the container) * `HEALTHCHECK NONE` (disable any healthcheck inherited from the base image) The `HEALTHCHECK` instruction tells Docker how to test a container to check that it is still working. This can detect cases such as a web server that is stuck in an infinite loop and unable to handle new connections, even though the server process is still running. When a container has a healthcheck specified, it has a _health status_ in addition to its normal status. This status is initially `starting`. Whenever a health check passes, it becomes `healthy` (whatever state it was previously in). After a certain number of consecutive failures, it becomes `unhealthy`. The options that can appear before `CMD` are: * `--interval=DURATION` (default: `30s`) * `--timeout=DURATION` (default: `30s`) * `--start-period=DURATION` (default: `0s`) * `--retries=N` (default: `3`) The health check will first run **interval** seconds after the container is started, and then again **interval** seconds after each previous check completes. If a single run of the check takes longer than **timeout** seconds then the check is considered to have failed. It takes **retries** consecutive failures of the health check for the container to be considered `unhealthy`. **start period** provides initialization time for containers that need time to bootstrap. Probe failure during that period will not be counted towards the maximum number of retries. However, if a health check succeeds during the start period, the container is considered started and all consecutive failures will be counted towards the maximum number of retries. There can only be one `HEALTHCHECK` instruction in a Dockerfile. If you list more than one then only the last `HEALTHCHECK` will take effect. The command after the `CMD` keyword can be either a shell command (e.g. `HEALTHCHECK CMD /bin/check-running`) or an _exec_ array (as with other Dockerfile commands; see e.g. `ENTRYPOINT` for details). The command's exit status indicates the health status of the container. The possible values are: - 0: success - the container is healthy and ready for use - 1: unhealthy - the container is not working correctly - 2: reserved - do not use this exit code For example, to check every five minutes or so that a web-server is able to serve the site's main page within three seconds: HEALTHCHECK --interval=5m --timeout=3s \ CMD curl -f http://localhost/ || exit 1 To help debug failing probes, any output text (UTF-8 encoded) that the command writes on stdout or stderr will be stored in the health status and can be queried with `docker inspect`. Such output should be kept short (only the first 4096 bytes are stored currently). When the health status of a container changes, a `health_status` event is generated with the new status. The `HEALTHCHECK` feature was added in Docker 1.12. ## SHELL SHELL ["executable", "parameters"] The `SHELL` instruction allows the default shell used for the *shell* form of commands to be overridden. The default shell on Linux is `["/bin/sh", "-c"]`, and on Windows is `["cmd", "/S", "/C"]`. The `SHELL` instruction *must* be written in JSON form in a Dockerfile. The `SHELL` instruction is particularly useful on Windows where there are two commonly used and quite different native shells: `cmd` and `powershell`, as well as alternate shells available including `sh`. The `SHELL` instruction can appear multiple times. Each `SHELL` instruction overrides all previous `SHELL` instructions, and affects all subsequent instructions. For example: FROM microsoft/windowsservercore # Executed as cmd /S /C echo default RUN echo default # Executed as cmd /S /C powershell -command Write-Host default RUN powershell -command Write-Host default # Executed as powershell -command Write-Host hello SHELL ["powershell", "-command"] RUN Write-Host hello # Executed as cmd /S /C echo hello SHELL ["cmd", "/S"", "/C"] RUN echo hello The following instructions can be affected by the `SHELL` instruction when the *shell* form of them is used in a Dockerfile: `RUN`, `CMD` and `ENTRYPOINT`. The following example is a common pattern found on Windows which can be streamlined by using the `SHELL` instruction: ... RUN powershell -command Execute-MyCmdlet -param1 "c:\foo.txt" ... The command invoked by docker will be: cmd /S /C powershell -command Execute-MyCmdlet -param1 "c:\foo.txt" This is inefficient for two reasons. First, there is an un-necessary cmd.exe command processor (aka shell) being invoked. Second, each `RUN` instruction in the *shell* form requires an extra `powershell -command` prefixing the command. To make this more efficient, one of two mechanisms can be employed. One is to use the JSON form of the RUN command such as: ... RUN ["powershell", "-command", "Execute-MyCmdlet", "-param1 \"c:\\foo.txt\""] ... While the JSON form is unambiguous and does not use the un-necessary cmd.exe, it does require more verbosity through double-quoting and escaping. The alternate mechanism is to use the `SHELL` instruction and the *shell* form, making a more natural syntax for Windows users, especially when combined with the `escape` parser directive: # escape=` FROM microsoft/nanoserver SHELL ["powershell","-command"] RUN New-Item -ItemType Directory C:\Example ADD Execute-MyCmdlet.ps1 c:\example\ RUN c:\example\Execute-MyCmdlet -sample 'hello world' Resulting in: PS E:\docker\build\shell> docker build -t shell . Sending build context to Docker daemon 4.096 kB Step 1/5 : FROM microsoft/nanoserver ---> 22738ff49c6d Step 2/5 : SHELL powershell -command ---> Running in 6fcdb6855ae2 ---> 6331462d4300 Removing intermediate container 6fcdb6855ae2 Step 3/5 : RUN New-Item -ItemType Directory C:\Example ---> Running in d0eef8386e97 Directory: C:\ Mode LastWriteTime Length Name ---- ------------- ------ ---- d----- 10/28/2016 11:26 AM Example ---> 3f2fbf1395d9 Removing intermediate container d0eef8386e97 Step 4/5 : ADD Execute-MyCmdlet.ps1 c:\example\ ---> a955b2621c31 Removing intermediate container b825593d39fc Step 5/5 : RUN c:\example\Execute-MyCmdlet 'hello world' ---> Running in be6d8e63fe75 hello world ---> 8e559e9bf424 Removing intermediate container be6d8e63fe75 Successfully built 8e559e9bf424 PS E:\docker\build\shell> The `SHELL` instruction could also be used to modify the way in which a shell operates. For example, using `SHELL cmd /S /C /V:ON|OFF` on Windows, delayed environment variable expansion semantics could be modified. The `SHELL` instruction can also be used on Linux should an alternate shell be required such as `zsh`, `csh`, `tcsh` and others. The `SHELL` feature was added in Docker 1.12. ## Dockerfile examples Below you can see some examples of Dockerfile syntax. If you're interested in something more realistic, take a look at the list of [Dockerization examples](https://docs.docker.com/engine/examples/). ``` # Nginx # # VERSION 0.0.1 FROM ubuntu LABEL Description="This image is used to start the foobar executable" Vendor="ACME Products" Version="1.0" RUN apt-get update && apt-get install -y inotify-tools nginx apache2 openssh-server ``` ``` # Firefox over VNC # # VERSION 0.3 FROM ubuntu # Install vnc, xvfb in order to create a 'fake' display and firefox RUN apt-get update && apt-get install -y x11vnc xvfb firefox RUN mkdir ~/.vnc # Setup a password RUN x11vnc -storepasswd 1234 ~/.vnc/passwd # Autostart firefox (might not be the best way, but it does the trick) RUN bash -c 'echo "firefox" >> /.bashrc' EXPOSE 5900 CMD ["x11vnc", "-forever", "-usepw", "-create"] ``` ``` # Multiple images example # # VERSION 0.1 FROM ubuntu RUN echo foo > bar # Will output something like ===> 907ad6c2736f FROM ubuntu RUN echo moo > oink # Will output something like ===> 695d7793cbe4 # You'll now have two images, 907ad6c2736f with /bar, and 695d7793cbe4 with # /oink. ```