moby--moby/docs/sources/reference/builder.md

page_title: Dockerfile Reference
page_description: Dockerfiles use a simple DSL which allows you to automate the steps you would normally manually take to create an image.
page_keywords: builder, docker, Dockerfile, automation, image creation

# 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 you would normally execute manually in order to build a
Docker image. By calling `docker build` from your terminal, you can have
Docker build your image step by step, executing the instructions
successively.

This page discusses the specifics of all the instructions you can use in your
`Dockerfile`. To further help you write a clear, readable, maintainable
`Dockerfile`, we've also written a [`Dockerfile` Best Practices
guide](/articles/dockerfile_best-practices). Lastly, you can test your
Dockerfile knowledge with the [Dockerfile tutorial](/userguide/level1).

## Usage

To [*build*](/reference/commandline/cli/#build) an image from a source repository,
create a description file called `Dockerfile` at the root of your repository.
This file will describe the steps to assemble the image.

Then call `docker build` with the path of your source repository as the argument
(for example, `.`):

    $ sudo docker build .

The path to the source repository defines where to find the *context* of
the build. The build is run by the Docker daemon, not by the CLI, so 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.

> **Warning**
> Avoid using your root directory, `/`, as the root of the source repository. The 
> `docker build` command will use whatever directory contains the Dockerfile as the build
> context (including all of its subdirectories). The build context will be sent to the
> Docker daemon before building the image, which means if you use `/` as the source
> repository, the entire contents of your hard drive will get sent to the daemon (and
> thus to the machine running the daemon). You probably don't want that.

In most cases, it's best to put each Dockerfile in an empty directory, and then add only
the files needed for building that Dockerfile to that directory. To further speed up the
build, you can exclude files and directories by adding a `.dockerignore` file to the same
directory.

You can specify a repository and tag at which to save the new image if
the build succeeds:

    $ sudo docker build -t shykes/myapp .

The Docker daemon will run your steps one-by-one, committing the result
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,
accelerating `docker build` significantly (indicated by `Using cache` -
see the [`Dockerfile` Best Practices
guide](/articles/dockerfile_best-practices/#build-cache) for more information):

    $ sudo docker build -t SvenDowideit/ambassador .
    Uploading context 10.24 kB
    Uploading context
    Step 1 : FROM docker-ut
     ---> cbba202fe96b
    Step 2 : MAINTAINER SvenDowideit@home.org.au
     ---> Using cache
     ---> 51182097be13
    Step 3 : CMD env | grep _TCP= | sed 's/.*_PORT_\([0-9]*\)_TCP=tcp:\/\/\(.*\):\(.*\)/socat TCP4-LISTEN:\1,fork,reuseaddr TCP4:\2:\3 \&/'  | sh && top
     ---> Using cache
     ---> 1a5ffc17324d
    Successfully built 1a5ffc17324d

When you're done with your build, you're ready to look into [*Pushing a
repository to its registry*]( /userguide/dockerrepos/#image-push).

## Format

Here is the format of the `Dockerfile`:

    # Comment
    INSTRUCTION arguments

The Instruction is not case-sensitive, however convention is for them to
be UPPERCASE in order to distinguish them from arguments more easily.

Docker runs the instructions in a `Dockerfile` in order. **The
first instruction must be \`FROM\`** in order to specify the [*Base
Image*](/terms/image/#base-image-def) from which you are building.

Docker will treat lines that *begin* with `#` as a
comment. A `#` marker anywhere else in the line will
be treated as an argument. This allows statements like:

    # Comment
    RUN echo 'we are running some # of cool things'

Here is the set of instructions you can use in a `Dockerfile` for building
images.

### Environment Replacement

**Note:** prior to 1.3, `Dockerfile` environment variables were handled
similarly, in that they would be replaced as described below. However, there
was no formal definition on as to which instructions handled environment
replacement at the time. After 1.3 this behavior will be preserved and
canonical.

Environment variables (declared with the `ENV` statement) 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`.

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

The instructions that handle environment variables in the `Dockerfile` are:

* `ENV`
* `ADD`
* `COPY`
* `WORKDIR`
* `EXPOSE`
* `VOLUME`
* `USER`

`ONBUILD` instructions are **NOT** supported for environment replacement, even
the instructions above.

## The `.dockerignore` file

If a file named `.dockerignore` exists in the source repository, then it
is interpreted as a newline-separated list of exclusion patterns.
Exclusion patterns match files or directories relative to the source repository
that will be excluded from the context. Globbing is done using Go's
[filepath.Match](http://golang.org/pkg/path/filepath#Match) rules.

The following example shows the use of the `.dockerignore` file to exclude the
`.git` directory from the context. Its effect can be seen in the changed size of
the uploaded context.

    $ sudo docker build .
    Uploading context 18.829 MB
    Uploading context
    Step 0 : FROM busybox
     ---> 769b9341d937
    Step 1 : CMD echo Hello World
     ---> Using cache
     ---> 99cc1ad10469
    Successfully built 99cc1ad10469
    $ echo ".git" > .dockerignore
    $ sudo docker build .
    Uploading context  6.76 MB
    Uploading context
    Step 0 : FROM busybox
     ---> 769b9341d937
    Step 1 : CMD echo Hello World
     ---> Using cache
     ---> 99cc1ad10469
    Successfully built 99cc1ad10469

## FROM

    FROM <image>

Or

    FROM <image>:<tag>

The `FROM` instruction sets the [*Base Image*](/terms/image/#base-image-def)
for subsequent instructions. As such, a valid `Dockerfile` must have `FROM` as
its first instruction. The image can be any valid image – it is especially easy
to start by **pulling an image** from the [*Public Repositories*](
/userguide/dockerrepos/#using-public-repositories).

`FROM` must be the first non-comment instruction in the `Dockerfile`.

`FROM` can appear multiple times within a single `Dockerfile` in order to create
multiple images. Simply 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, `latest` is assumed. If the
used tag does not exist, an error will be returned.

## MAINTAINER

    MAINTAINER <name>

The `MAINTAINER` instruction allows you to set the *Author* field of the
generated images.

## RUN

RUN has 2 forms:

- `RUN <command>` (the command is run in a shell - `/bin/sh -c` - *shell* form)
- `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 `/bin/sh`.

> **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" ]`.

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](/articles/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. 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 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 `<command>` will execute in
`/bin/sh -c`:

    FROM ubuntu
    CMD echo "This is a test." | wc -

If you want to **run your** `<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 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.

## EXPOSE

    EXPOSE <port> [<port>...]

The `EXPOSE` instructions informs Docker that the container will listen on the
specified network ports at runtime. Docker uses this information to interconnect
containers using links (see the [Docker User
Guide](/userguide/dockerlinks)) and to determine which ports to expose to the
host when [using the -P flag](/reference/run/#expose-incoming-ports).
**Note:**
`EXPOSE` doesn't define which ports can be exposed to the host or make ports
accessible from the host by default. To expose ports to the host, at runtime, 
[use the `-p` flag](/userguide/dockerlinks) or
[the -P flag](/reference/run/#expose-incoming-ports).

## ENV

    ENV <key> <value>
    ENV <key>=<value> ...

The `ENV` instruction sets the environment variable `<key>` to the value
`<value>`. This value will be passed to all future `RUN` instructions. This is
functionally equivalent to prefixing the command with `<key>=<value>`

The `ENV` instruction has two forms. The first form, `ENV <key> <value>`,
will set a single variable to a value. The entire string after the first
space will be treated as the `<value>` - including characters such as 
spaces and quotes.

The second form, `ENV <key>=<value> ...`, 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 container, but the first form 
does it all in one 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 <key>=<value>`.

> **Note**:
> One example where this can cause unexpected consequences, is setting
> `ENV DEBIAN_FRONTEND noninteractive`. Which will persist when the container
> is run interactively; for example: `docker run -t -i image bash`

## ADD

    ADD <src>... <dest>

The `ADD` instruction copies new files, directories or remote file URLs from `<src>`
and adds them to the filesystem of the container at the path `<dest>`.  

Multiple `<src>` 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 `<src>` may contain wildcards and matching will be done using Go's
[filepath.Match](http://golang.org/pkg/path/filepath#Match) rules.
For most command line uses this should act as expected, for example:

    ADD hom* /mydir/        # adds all files starting with "hom"
    ADD hom?.txt /mydir/    # ? is replaced with any single character

The `<dest>` is the absolute path to which the source will be copied inside the
destination container.

All new files and directories are created with a UID and GID of 0.

In the case where `<src>` 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. Then, like any other file
processed during an `ADD`, `mtime` will 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 get used at 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 `<src>` have
> changed. This includes invalidating the cache for `RUN` instructions.
> See the [`Dockerfile` Best Practices
guide](/articles/dockerfile_best-practices/#build-cache) for more information.


The copy obeys the following rules:

- The `<src>` 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 `<src>` is a URL and `<dest>` does not end with a trailing slash, then a
  file is downloaded from the URL and copied to `<dest>`.

- If `<src>` is a URL and `<dest>` does end with a trailing slash, then the
  filename is inferred from the URL and the file is downloaded to
  `<dest>/<filename>`. 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 `<src>` 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 `<src>` 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.

- If `<src>` is any other kind of file, it is copied individually along with
  its metadata. In this case, if `<dest>` ends with a trailing slash `/`, it
  will be considered a directory and the contents of `<src>` will be written
  at `<dest>/base(<src>)`.

- If multiple `<src>` resources are specified, either directly or due to the
  use of a wildcard, then `<dest>` must be a directory, and it must end with 
  a slash `/`.

- If `<dest>` does not end with a trailing slash, it will be considered a
  regular file and the contents of `<src>` will be written at `<dest>`.

- If `<dest>` doesn't exist, it is created along with all missing directories
  in its path.

## COPY

    COPY <src>... <dest>

The `COPY` instruction copies new files or directories from `<src>`
and adds them to the filesystem of the container at the path `<dest>`.

Multiple `<src>` resource may be specified but they must be relative
to the source directory that is being built (the context of the build).

Each `<src>` may contain wildcards and matching will be done using Go's
[filepath.Match](http://golang.org/pkg/path/filepath#Match) rules.
For most command line uses this should act as expected, for example:

    COPY hom* /mydir/        # adds all files starting with "hom"
    COPY hom?.txt /mydir/    # ? is replaced with any single character

The `<dest>` is the absolute path to which the source will be copied inside the
destination container.

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.

The copy obeys the following rules:

- The `<src>` 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 `<src>` 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 `<src>` is any other kind of file, it is copied individually along with
  its metadata. In this case, if `<dest>` ends with a trailing slash `/`, it
  will be considered a directory and the contents of `<src>` will be written
  at `<dest>/base(<src>)`.

- If multiple `<src>` resources are specified, either directly or due to the
  use of a wildcard, then `<dest>` must be a directory, and it must end with 
  a slash `/`.

- If `<dest>` does not end with a trailing slash, it will be considered a
  regular file and the contents of `<src>` will be written at `<dest>`.

- If `<dest>` doesn't exist, it is created along with all missing directories
  in its path.

## ENTRYPOINT

ENTRYPOINT has two forms:

- `ENTRYPOINT ["executable", "param1", "param2"]`
  (the preferred *exec* form)
- `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 <image>` 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 <image> -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 <container>`.

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`
(see [the Dockerfile best practices](/articles/dockerfile_best-practices/#entrypoint)
for more details):

```bash
#!/bin/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 KILL TERM

# start service in background here
/usr/sbin/apachectl start

echo "[hit enter key to exit] or run 'docker stop <container>'"
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 over ride 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" ]`.
> Variables that are defined in the `Dockerfile`using `ENV`, will be substituted by
> the `Dockerfile` parser.

### 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

## VOLUME

    VOLUME ["/data"]

The `VOLUME` instruction will create a mount point with the specified name
and mark 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*](/userguide/dockervolumes/#volume-def)
documentation.

> **Note**:
> The list is parsed as a JSON array, which means that
> you must use double-quotes (") around words not single-quotes (').

## 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` and
`ENTRYPOINT` instructions that follow it in the `Dockerfile`.

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

The output of the final `pwd` command in this `Dockerfile` would be
`/path/$DIRNAME`

## 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.

## Dockerfile Examples

    # Nginx
    #
    # VERSION               0.0.1

    FROM      ubuntu
    MAINTAINER Victor Vieux <victor@docker.com>

    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.