moby--moby/docs/sources/articles/dockerfile_best-practices.md

page_title: Best Practices for Writing Dockerfiles
page_description: Hints, tips and guidelines for writing clean, reliable Dockerfiles
page_keywords: Examples, Usage, base image, docker, documentation, dockerfile, best practices, hub, official repo

# Best Practices for Writing `Dockerfile`s

## Overview

Docker can build images automatically by reading the instructions from a
`Dockerfile`, a text file that contains all the commands, in order, needed to
build a given image. `Dockerfile`s adhere to a specific format and use a
specific set of instructions. You can learn the basics on the 
[Dockerfile Reference](https://docs.docker.com/reference/builder/) page. If
you’re new to writing `Dockerfile`s, you should start there.

This document covers the best practices and methods recommended by Docker,
Inc. and the Docker Community for creating easy-to-use, effective
`Dockerfile`s. We strongly suggest you follow these recommendations (in fact,
if you’re creating an Official Image, you *must* adhere to these practices).

You can see many of these practices and recommendations in action in the [buildpack-deps `Dockerfile`](https://github.com/docker-library/buildpack-deps/blob/master/jessie/Dockerfile).

> Note: for more detailed explanations of any of the Dockerfile commands
>mentioned here, visit the [Dockerfile Reference](https://docs.docker.com/reference/builder/) page.

## General Guidelines and Recommendations

### Containers should be ephemeral

The container produced by the image your `Dockerfile` defines should be as
ephemeral as possible. “Ephemeral” here means that it can be stopped and
destroyed and a new one built and put in place with an absolute minimum of
set-up and configuration.

### Use a [`.dockerignore` file](https://docs.docker.com/reference/builder/#the-dockerignore-file)

For faster uploading and efficiency during `docker build`, you should make use
of a `.dockerignore` file to exclude files or directories from the build
context and final image. For example, unless`.git` is needed by your build
process or scripts, you should add it to `.dockerignore`, which can save many
megabytes worth of upload time.

### Avoid installing unnecessary packages

You should avoid installing extra or unnecessary packages just because they
might be “nice to have.” For example, you don’t need to include a text editor
in a database image.

### Run only one process per container

In almost all cases, you should only run a single process in a single
container. Decoupling applications into multiple containers makes it much
easier to scale horizontally and reuse containers. If that service depends on
another service, make use of [container linking](https://docs.docker.com/userguide/dockerlinks/).

### Minimize the number of layers

You need to find the balance between readability (and thus long-term
maintainability) of the `Dockerfile` and minimizing the number of layers it
uses. Be strategic and cautious about the number of layers you use.

### Sort multi-line arguments

Whenever possible, ease later changes by sorting multi-line arguments
alphanumerically. This will help you avoid duplication of packages and make the
list much easier to update. This also makes PRs a lot easier to read and
review. Adding a space before a backslash (`\`) helps as well. 

Here’s an example from the [`buildpack-deps` image](https://github.com/docker-library/buildpack-deps):

    RUN apt-get update && apt-get install -y \
      bzr \
      cvs \
      git \
      mercurial \
      subversion

## The `Dockerfile` instructions

This section contains specific recommendations for the correct usage of the
various instructions contained in a `Dockerfile`.

### [`FROM`](https://docs.docker.com/reference/builder/#from)

Whenever possible, use Official Repositories as the basis for your image. We
recommend the [Debian image](https://registry.hub.docker.com/_/debian/) since
it’s very tightly controlled and kept extremely minimal (currently under 100
mb), while still being a full distribution.

### [`RUN`](https://docs.docker.com/reference/builder/#run)

As always, to make your `Dockerfile` more readable, understandable, and
maintainable, put long or complex `RUN` statements on multiple lines separated with
backslashes.

Probably the most common use-case for `RUN` is an application of `apt-get`.
When using `apt-get`, here a few things to keep in mind:

* Don’t do `RUN apt-get update` on a single line. This will cause
caching issues if the referenced archive gets updated, which will make your
subsequent `apt-get install` fail without comment.

* For the most part, to keep your code more readable and maintainable, avoid
`RUN apt-get install -y package-foo && apt-get install -y package-bar`.

* Avoid `RUN apt-get upgrade` or `dist-upgrade`, since many of the “essential”
packages from the base images will fail to upgrade inside an unprivileged
container. If a base package is out of date, you should contact its
maintainers. If you know there’s a particular package, `foo`, that needs to be
updated, use `apt-get install -y foo` and it will update automatically.

* Do use `RUN apt-get update && apt-get install -y package-bar package-foo
package-baz`. Writing the instruction this way not only makes it easier to read
and maintain, but also, by including `apt-get update`, ensures that the cache
will naturally be busted and the latest versions will be installed with no
further coding or manual intervention required.

* Further natural cache-busting can be realized by version-pinning packages
(e.g., `package-foo=1.3.*`). This will force retrieval of that version
regardless of what’s in the cache.
Forming your `apt-get` code this way will greatly ease maintenance and reduce
failures due to unanticipated changes in required packages.

#### Example

Below is a well-formed `RUN` instruction that demonstrates the above
recommendations. Note that the last package, `s3cmd`, specifies a version
`1.1.0*`. If the image previously used an older version, specifying the new one
will cause a cache bust of `apt-get update` and ensure the installation of
the new version (which in this case had a new, required feature).

    RUN apt-get update && apt-get install -y \
        aufs-tools \
        automake \
        btrfs-tools \
        build-essential \
        curl \
        dpkg-sig \
        git \
        iptables \
        libapparmor-dev \
        libcap-dev \
        libsqlite3-dev \
        lxc=1.0* \
        mercurial \
        parallel \
        reprepro \
        ruby1.9.1 \
        ruby1.9.1-dev \
        s3cmd=1.1.0*

### [`CMD`](https://docs.docker.com/reference/builder/#cmd)

The `CMD` instruction should be used to run the software contained by your
image, along with any arguments. `CMD` should almost always be used in the
form of `CMD [“executable”, “param1”, “param2”…]`. Thus, if the image is for a
service (Apache, Rails, etc.), you would run something like
`CMD ["apache2","-DFOREGROUND"]`. Indeed, this form of the instruction is
recommended for any service-based image.

In most other cases, `CMD` should be given an interactive shell (bash, python,
perl, etc), for example, `CMD ["perl", "-de0"]`, `CMD ["python"]`, or
`CMD [“php”, “-a”]`. Using this form means that when you execute something like
`docker run -it python`, you’ll get dropped into a usable shell, ready to go.
`CMD` should rarely be used in the manner of `CMD [“param”, “param”]` in
conjunction with [`ENTRYPOINT`](https://docs.docker.com/reference/builder/#entrypoint), unless
you and your expected users are already quite familiar with how `ENTRYPOINT`
works. 

### [`EXPOSE`](https://docs.docker.com/reference/builder/#expose)

The `EXPOSE` instruction indicates the ports on which a container will listen
for connections. Consequently, you should use the common, traditional port for
your application. For example, an image containing the Apache web server would
use `EXPOSE 80`, while an image containing MongoDB would use `EXPOSE 27017` and
so on.

For external access, your users can execute `docker run` with a flag indicating
how to map the specified port to the port of their choice.
For container linking, Docker provides environment variables for the path from
the recipient container back to the source (ie, `MYSQL_PORT_3306_TCP`).

### [`ENV`](https://docs.docker.com/reference/builder/#env)

In order to make new software easier to run, you can use `ENV` to update the
`PATH` environment variable for the software your container installs. For
example, `ENV PATH /usr/local/nginx/bin:$PATH` will ensure that `CMD [“nginx”]`
just works.

The `ENV` instruction is also useful for providing required environment
variables specific to services you wish to containerize, such as Postgres’s
`PGDATA`.

Lastly, `ENV` can also be used to set commonly used version numbers so that
version bumps are easier to maintain, as seen in the following example:

    ENV PG_MAJOR 9.3
    ENV PG_VERSION 9.3.4
    RUN curl -SL http://example.com/postgres-$PG_VERSION.tar.xz | tar -xJC /usr/src/postgress && …
    ENV PATH /usr/local/postgres-$PG_MAJOR/bin:$PATH

Similar to having constant variables in a program (as opposed to hard-coding
values), this approach lets you change a single `ENV` instruction to
auto-magically bump the version of the software in your container.

### [`ADD`](https://docs.docker.com/reference/builder/#add) or [`COPY`](https://docs.docker.com/reference/builder/#copy)

Although `ADD` and `COPY` are functionally similar, generally speaking, `COPY`
is preferred. That’s because it’s more transparent than `ADD`. `COPY` only
supports the basic copying of local files into the container, while `ADD` has
some features (like local-only tar extraction and remote URL support) that are
not immediately obvious. Consequently, the best use for `ADD` is local tar file
auto-extraction into the image, as in `ADD rootfs.tar.xz /`.

Because image size matters, using `ADD` to fetch packages from remote URLs is
strongly discouraged; you should use `curl` or `wget` instead. That way you can
delete the files you no longer need after they’ve been extracted and you won't
have to add another layer in your image. For example, you should avoid doing
things like:

    ADD http://example.com/big.tar.xz /usr/src/things/
    RUN tar -xJf /usr/src/things/big.tar.xz -C /usr/src/things
    RUN make -C /usr/src/things all

And instead, do something like:

    RUN mdkir -p /usr/src/things \
        && curl -SL http://example.com/big.tar.gz \
        | tar -xJC /usr/src/things \
        && make -C /usr/src/things all

For other items (files, directories) that do not require `ADD`’s tar
auto-extraction capability, you should always use `COPY`.

### [`ENTRYPOINT`](https://docs.docker.com/reference/builder/#entrypoint)

The best use for `ENTRYPOINT` is as a helper script. Using `ENTRYPOINT` for
other tasks can make your code harder to understand. For example,
`docker run -it official-image bash` is much easier to understand than
`docker run -it --entrypoint bash official-image -i`, especially for Docker
beginners.

In order to avoid a situation where commands are run without clear visibility
to the user, make sure your script ends with something like `exec "$@"`. After
the entrypoint completes, the script will transparently bootstrap the command
invoked by the user, making what has been run clear to the user (for example,
`docker run -it mysql mysqld --some --flags` will transparently run
`mysqld --some --flags` after `ENTRYPOINT` runs `initdb`).

For example, let’s look at the `Dockerfile` for the
[Postgres Official Image](https://github.com/docker-library/postgres).
It refers to the following script: 

```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 "$@"
```

That script then gets copied into the container and the run via `ENTRYPOINT` on
container startup: 

    COPY ./docker-entrypoint.sh /
    ENTRYPOINT ["/docker-entrypoint.sh"]

### [`VOLUME`](https://docs.docker.com/reference/builder/#volume)

The `VOLUME` instruction should be used to expose any database storage area,
configuration storage, or files/folders created by your docker container. You
are strongly encouraged to use `VOLUME` for any mutable and/or user-serviceable
parts of your image.

### [`USER`](https://docs.docker.com/reference/builder/#user)

If a service can run without privileges, use `USER` to change to a non-root
user. Start by creating the user and group in the `Dockerfile` with something
like `RUN groupadd -r postgres && useradd -r -g postgres postgres`.

>**Note** that users/groups in an image get assigned a non-deterministic
>UID/GID in that the “next” UID/GID gets assigned regardless of image
>rebuilds. So, if it’s critical, you should assign an explicit UID/GID.

You should avoid installing or using `sudo` since it has unpredictable TTY and
signal-forwarding behavior that can cause more more problems than it solves. If
you absolutely need functionality similar to `sudo` (e.g., initializing the
daemon as root but running it as non-root), you may be able to use
[“gosu”](https://github.com/tianon/gosu). 

Lastly, to reduce layers and complexity, try to minimize switching `USER` back
and forth frequently.

### [`WORKDIR`](https://docs.docker.com/reference/builder/#workdir)

For clarity and reliability, you should always use absolute paths for your
`WORKDIR`. Also, you should use `WORKDIR` instead of  proliferating
instructions like `RUN cd … && do-something`, which are hard to read,
troubleshoot, and maintain.

### [`ONBUILD`](https://docs.docker.com/reference/builder/#onbuild)

`ONBUILD` is only useful for images that are going to be built `FROM` a given
image. For example, you would use `ONBUILD` for a language stack image that
builds arbitrary user software written in that language within the
`Dockerfile`, as you can see in [Ruby’s `ONBUILD` variants](https://github.com/docker-library/ruby/blob/master/2.1/onbuild/Dockerfile). 

Images built from `ONBUILD` should get a separate tag, for example:
`ruby:1.9-onbuild` or `ruby:2.0-onbuild`.

Be careful when putting `ADD` or `COPY` in `ONBUILD`. The “onbuild” image will
fail catastrophically if the new build's context is missing the resource being
added. Adding a separate tag, as recommended above, will help mitigate this by
allowing the `Dockerfile` author to make a choice.

## Examples For Official Repositories

These Official Repos have exemplary `Dockerfile`s:

* [Go](https://registry.hub.docker.com/_/golang/)
* [Perl](https://registry.hub.docker.com/_/perl/)
* [Hy](https://registry.hub.docker.com/_/hylang/)
* [Rails](https://registry.hub.docker.com/_/rails)

## Additional Resources:

* [Dockerfile Reference](https://docs.docker.com/reference/builder/#onbuild)
* [More about Base Images](https://docs.docker.com/articles/baseimages/)
* [More about Automated Builds](https://docs.docker.com/docker-hub/builds/)
* [Guidelines for Creating Official Repositories](https://docs.docker.com/docker-hub/official_repos/)