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% DOCKERFILE(5) Docker User Manuals
% Zac Dover
% May 2014
# NAME
Dockerfile - automate the steps of creating a Docker image
# INTRODUCTION
The **Dockerfile** is a configuration file that automates the steps of creating
a Docker image. It is similar to a Makefile. Docker reads instructions from the
**Dockerfile** to automate the steps otherwise performed manually to create an
image. To build an image, create a file called **Dockerfile**.
The **Dockerfile** describes the steps taken to assemble the image. When the
**Dockerfile** has been created, call the `docker build` command, using the
path of directory that contains **Dockerfile** as the argument.
# SYNOPSIS
INSTRUCTION arguments
For example:
FROM image
# DESCRIPTION
A Dockerfile is a file that automates the steps of creating a Docker image.
A Dockerfile is similar to a Makefile.
# USAGE
docker build .
-- Runs the steps and commits them, building a final image.
The path to the source repository defines where to find the context of the
build. The build is run by the Docker daemon, not the CLI. The whole
context must be transferred to the daemon. The Docker CLI reports
`"Sending build context to Docker daemon"` when the context is sent to the
daemon.
```
docker build -t repository/tag .
```
-- specifies a repository and tag at which to save the new image if the build
succeeds. The Docker daemon runs the steps one-by-one, committing the result
to a new image if necessary, before finally outputting the ID of the new
image. The Docker daemon automatically cleans up the context it is given.
Docker re-uses intermediate images whenever possible. This significantly
accelerates the *docker build* process.
# FORMAT
`FROM image`
`FROM image:tag`
`FROM image@digest`
-- The **FROM** instruction sets the base image for subsequent instructions. A
valid Dockerfile must have **FROM** as its first instruction. The image can be any
valid image. It is easy to start by pulling an image from the public
repositories.
-- **FROM** must be the first non-comment instruction in Dockerfile.
-- **FROM** may appear multiple times within a single Dockerfile in order to create
multiple images. Make a note of the last image ID output by the commit before
each new **FROM** command.
-- If no tag is given to the **FROM** instruction, Docker applies the
`latest` tag. If the used tag does not exist, an error is returned.
-- If no digest is given to the **FROM** instruction, Docker applies the
`latest` tag. If the used tag does not exist, an error is returned.
**MAINTAINER**
-- **MAINTAINER** sets the Author field for the generated images.
Useful for providing users with an email or url for support.
**RUN**
-- **RUN** has two forms:
```
# the command is run in a shell - /bin/sh -c
RUN <command>
# Executable form
RUN ["executable", "param1", "param2"]
```
-- The **RUN** instruction executes any commands in a new layer on top of the current
image and commits the results. The committed image is used for the next step in
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 the history of an image. This is similar to source control. The
exec form makes it possible to avoid shell string munging. The exec form makes
it possible to **RUN** commands using a base image that does not contain `/bin/sh`.
Note that the exec form is parsed as a JSON array, which means that you must
use double-quotes (") around words not single-quotes (').
**CMD**
-- **CMD** has three forms:
```
# Executable form
CMD ["executable", "param1", "param2"]`
# Provide default arguments to ENTRYPOINT
CMD ["param1", "param2"]`
# the command is run in a shell - /bin/sh -c
CMD command param1 param2
```
-- There should be only one **CMD** in a Dockerfile. If more than one **CMD** is listed, only
the last **CMD** takes effect.
The main purpose of a **CMD** is to provide defaults for an executing container.
These defaults may include an executable, or they can omit the executable. If
they omit the executable, an **ENTRYPOINT** must be specified.
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**, the `<command>` executes in `/bin/sh -c`:
Note that the exec form is parsed as a JSON array, which means that you must
use double-quotes (") around words not single-quotes (').
```
FROM ubuntu
CMD echo "This is a test." | wc -
```
-- If you run **command** 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 form of **CMD**. All additional parameters must be individually expressed
as strings in the array:
```
FROM ubuntu
CMD ["/usr/bin/wc","--help"]
```
-- To make the container run the same executable every time, use **ENTRYPOINT** in
combination with **CMD**.
If the user specifies arguments to `docker run`, the specified commands
override the default in **CMD**.
Do not confuse **RUN** with **CMD**. **RUN** runs a command and commits the result.
**CMD** executes nothing at build time, but specifies the intended command for
the image.
**LABEL**
-- `LABEL <key>[=<value>] [<key>[=<value>] ...]`or
```
LABEL <key>[ <value>]
LABEL <key>[ <value>]
...
```
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.
```
LABEL com.example.vendor="ACME Incorporated"
or
LABEL com.example.vendor "ACME Incorporated"
```
An image can have more than one label. To specify multiple labels, separate
each key-value pair by a space.
Labels are additive including `LABEL`s in `FROM` images. As the system
encounters and then applies a new label, new `key`s override any previous
labels with identical keys.
To display an image's labels, use the `docker inspect` command.
**EXPOSE**
-- `EXPOSE <port> [<port>...]`
The **EXPOSE** instruction informs Docker that the container listens on the
specified network ports at runtime. Docker uses this information to
interconnect containers using links, and to set up port redirection on the host
system.
**ENV**
-- `ENV <key> <value>`
The **ENV** instruction sets the environment variable <key> to
the value `<value>`. This value is passed to all future
**RUN**, **ENTRYPOINT**, and **CMD** instructions. This is
functionally equivalent to prefixing the command with `<key>=<value>`. The
environment variables that are set with **ENV** persist when a container is run
from the resulting image. Use `docker inspect` to inspect these values, and
change them using `docker run --env <key>=<value>`.
Note that setting "`ENV DEBIAN_FRONTEND noninteractive`" may cause
unintended consequences, because it will persist when the container is run
interactively, as with the following command: `docker run -t -i image bash`
**ADD**
-- **ADD** has two forms:
```
ADD <src> <dest>
# Required for paths with whitespace
ADD ["<src>",... "<dest>"]
```
The **ADD** instruction copies new files, directories
or remote file URLs to the filesystem of the container at path `<dest>`.
Multiple `<src>` resources 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). The `<dest>` is the absolute path, or path relative
to **WORKDIR**, into which the source is copied inside the target container.
If the `<src>` argument is a local file in a recognized compression format
(tar, gzip, bzip2, etc) then it is unpacked at the specified `<dest>` in the
container's filesystem. Note that only local compressed files will be unpacked,
i.e., the URL download and archive unpacking features cannot be used together.
All new directories are created with mode 0755 and with the uid and gid of **0**.
**COPY**
-- **COPY** has two forms:
```
COPY <src> <dest>
# Required for paths with whitespace
COPY ["<src>",... "<dest>"]
```
The **COPY** instruction copies new files from `<src>` and
adds them to the filesystem of the container at path <dest>. The `<src>` must be
the path to a file or directory relative to the source directory that is
being built (the context of the build) or a remote file URL. The `<dest>` is an
absolute path, or a path relative to **WORKDIR**, into which the source will
be copied inside the target container. If you **COPY** an archive file it will
land in the container exactly as it appears in the build context without any
attempt to unpack it. All new files and directories are created with mode **0755**
and with the uid and gid of **0**.
**ENTRYPOINT**
-- **ENTRYPOINT** has two forms:
```
# executable form
ENTRYPOINT ["executable", "param1", "param2"]`
# run command in a shell - /bin/sh -c
ENTRYPOINT command param1 param2
```
-- An **ENTRYPOINT** helps you configure a
container that can be run as an executable. When you specify an **ENTRYPOINT**,
the whole container runs as if it was only that executable. The **ENTRYPOINT**
instruction adds an entry command that is not overwritten when arguments are
passed to docker run. This is different from the behavior of **CMD**. This allows
arguments to be passed to the entrypoint, for instance `docker run <image> -d`
passes the -d argument to the **ENTRYPOINT**. Specify parameters either in the
**ENTRYPOINT** JSON array (as in the preferred exec form above), or by using a **CMD**
statement. Parameters in the **ENTRYPOINT** are not overwritten by the docker run
arguments. Parameters specified via **CMD** are overwritten by docker run
arguments. Specify a plain string for the **ENTRYPOINT**, and it will execute in
`/bin/sh -c`, like a **CMD** instruction:
```
FROM ubuntu
ENTRYPOINT wc -l -
```
This means that the Dockerfile's image always takes stdin as input (that's
what "-" means), and prints the number of lines (that's what "-l" means). To
make this optional but default, use a **CMD**:
```
FROM ubuntu
CMD ["-l", "-"]
ENTRYPOINT ["/usr/bin/wc"]
```
**VOLUME**
-- `VOLUME ["/data"]`
The **VOLUME** instruction creates a mount point with the specified name and marks
it as holding externally-mounted volumes from the native host or from other
containers.
**USER**
-- `USER daemon`
Sets the username or UID used for running subsequent commands.
The **USER** instruction can optionally be used to set the group or GID. The
followings examples are all valid:
USER [user | user:group | uid | uid:gid | user:gid | uid:group ]
Until the **USER** instruction is set, instructions will be run as root. The USER
instruction can be used any number of times in a Dockerfile, and will only affect
subsequent commands.
**WORKDIR**
-- `WORKDIR /path/to/workdir`
The **WORKDIR** instruction sets the working directory for the **RUN**, **CMD**,
**ENTRYPOINT**, **COPY** and **ADD** Dockerfile commands that follow it. It can
be used multiple times in a single Dockerfile. Relative paths are defined
relative to the path of the previous **WORKDIR** instruction. For example:
```
WORKDIR /a
WORKDIR b
WORKDIR c
RUN pwd
```
In the above example, the output of the **pwd** command is **a/b/c**.
**ARG**
-- ARG <name>[=<default value>]
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
<varname>=<value>` flag. If a user specifies a build argument that was not
defined in the Dockerfile, the build outputs an error.
```
One or more build-args were not consumed, failing build.
```
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 Dockerfile
```
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.
> **Note:** It is not recommended to use build-time variables for
> passing secrets like github keys, user credentials etc.
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 Dockerfile
```
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 Dockerfile
```
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 `--build-arg
<varname>=<value>` flag.
**ONBUILD**
-- `ONBUILD [INSTRUCTION]`
The **ONBUILD** instruction adds a trigger instruction to an image. The
trigger is executed at a later time, when the image is used as the base for
another build. Docker executes the trigger in the context of the downstream
build, as if the trigger existed immediately after the **FROM** instruction in
the downstream Dockerfile.
You can register any build instruction as a trigger. A trigger is useful if
you are defining an image to use as a base for building other images. For
example, if you are defining an application build environment or a daemon that
is customized with a user-specific configuration.
Consider an image intended as a reusable python application builder. It must
add application source code to a particular directory, and might need a build
script 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 is different
for each application build.
-- Providing application developers with a boilerplate Dockerfile to copy-paste
into their application is inefficient, error-prone, and
difficult to update because it mixes with application-specific code.
The solution is to use **ONBUILD** to register instructions in advance, to
run later, during the next build stage.
# HISTORY
*May 2014, Compiled by Zac Dover (zdover at redhat dot com) based on docker.com Dockerfile documentation.
*Feb 2015, updated by Brian Goff (cpuguy83@gmail.com) for readability
*Sept 2015, updated by Sally O'Malley (somalley@redhat.com)