:title: Docker Builder
:description: Docker Builder specifes a simple DSL which allows you to automate the steps you would normally manually take to create an image.
:keywords: builder, docker, Docker Builder, automation, image creation

==============
Docker Builder
==============

.. contents:: Table of Contents

Docker Builder specifes a simple DSL which allows you to automate the steps you
would normally manually take to create an image. Docker Build will run your 
steps and commit them along the way, giving you a final image.

1. Usage
========

To 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 argument:

    ``docker build .``

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

    ``docker build -t shykes/myapp .``

Docker will run your steps one-by-one, committing the result if necessary, 
before finally outputting the ID of your new image.

2. Format
=========

The Dockerfile format is quite simple:

    ``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.

Dockerfiles are evaluated in order, therefore the first instruction must be 
`FROM` in order to specify the base image from which you are building.

Docker will ignore lines in Dockerfiles prefixed with "`#`", so you may add 
comment lines. A comment marker in the rest of the line will be treated as an
argument.

3. Instructions
===============

Docker builder comes with a set of instructions, described below.

3.1 FROM
--------

    ``FROM <image>``

The `FROM` instruction sets the base image for subsequent instructions. As such,
a valid Dockerfile must have it as its first instruction.

`FROM` can be included 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.

3.2 MAINTAINER
--------------

    ``MAINTAINER <name>``

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

3.3 RUN
-------

    ``RUN <command>``

The `RUN` instruction will execute any commands on 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.

3.4 CMD
-------

    ``CMD <command>``

The `CMD` instruction sets the command to be executed when running the image.
This is functionally equivalent to running 
`docker commit -run '{"Cmd": <command>}'` outside the builder.

.. 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.

3.5 EXPOSE
----------

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

The `EXPOSE` instruction sets ports to be publicly exposed when running the 
image. This is functionally equivalent to running 
`docker commit -run '{"PortSpecs": ["<port>", "<port2>"]}'` outside the builder.

3.6 ENV
-------

    ``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>`

.. note::
    The environment variables will persist when a container is run from the resulting image.

3.7 ADD
-------

    ``ADD <src> <dest>``

The `ADD` instruction will copy new files from <src> and add them to the container's filesystem at path `<dest>`.

`<src>` must be the path to a file or directory relative to the source directory being built (also called the
context of the build) or a remote file URL.

`<dest>` is the path at which the source will be copied in the destination container.

The copy obeys the following rules:

If `<src>` is a directory, the entire directory is copied, including filesystem metadata.

If `<src>` is a tar archive in a recognized compression format (identity, gzip, bzip2 or xz), it
is unpacked as a directory.

When a directory is copied or unpacked, it has the same behavior as 'tar -x': the result is the union of
a) whatever existed at the destination path and b) the contents of the source tree, with conflicts resolved
in favor of b 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 `<dst>` ends with a trailing slash '/', it will be considered a directory and the contents of `<src>`
will be written at `<dst>/base(<src>)`.
If `<dst>` does not end with a trailing slash, it will be considered a regular file and the contents
of `<src>` will be written at `<dst>`.

If `<dest>` doesn't exist, it is created along with all missing directories in its path. All new
files and directories are created with mode 0700, uid and gid 0.

3.8 ENTRYPOINT
-------------

    ``ENTRYPOINT /bin/echo``

The `ENTRYPOINT` instruction adds an entry command that will not be overwritten when arguments are passed to docker run, unlike the behavior of `CMD`.  This allows arguments to be passed to the entrypoint.  i.e. `docker run <image> -d` will pass the "-d" argument to the entrypoint.

3.9 VOLUME
----------

    ``VOLUME ["/data"]``

The `VOLUME` instruction will add one or more new volumes to any container created from the image.

4. Dockerfile Examples
======================

.. code-block:: bash

    # Nginx
    #
    # VERSION               0.0.1
    
    FROM      ubuntu
    MAINTAINER Guillaume J. Charmes "guillaume@dotcloud.com"
    
    # make sure the package repository is up to date
    RUN echo "deb http://archive.ubuntu.com/ubuntu precise main universe" > /etc/apt/sources.list
    RUN apt-get update
    
    RUN apt-get install -y inotify-tools nginx apache2 openssh-server

.. code-block:: bash

    # Firefox over VNC
    #
    # VERSION               0.3
    
    FROM ubuntu
    # make sure the package repository is up to date
    RUN echo "deb http://archive.ubuntu.com/ubuntu precise main universe" > /etc/apt/sources.list
    RUN apt-get update
    
    # Install vnc, xvfb in order to create a 'fake' display and firefox
    RUN 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"]

.. code-block:: bash

    # 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.