General best practices for writing Dockerfiles

Use multi-stage builds

Multi-stage builds let you reduce the size of your final image, by creating a cleaner separation between the building of your image and the final output. Split your Dockerfile instructions into distinct stages to make sure that the resulting output only contains the files that's needed to run the application.

Using multiple stages can also let you build more efficiently by executing build steps in parallel.

See Multi-stage builds for more information.

Exclude with .dockerignore

To exclude files not relevant to the build, without restructuring your source repository, use a .dockerignore file. This file supports exclusion patterns similar to .gitignore files. For information on creating one, see Dockerignore file.

Create ephemeral containers

The image defined by your Dockerfile should generate containers that are as ephemeral as possible. Ephemeral means that the container can be stopped and destroyed, then rebuilt and replaced with an absolute minimum set up and configuration.

Refer to Processesopen_in_new under The Twelve-factor App methodology to get a feel for the motivations of running containers in such a stateless fashion.

Don't install unnecessary packages

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.

When you avoid installing extra or unnecessary packages, your images have reduced complexity, reduced dependencies, reduced file sizes, and reduced build times.

Decouple applications

Each container should have only one concern. Decoupling applications into multiple containers makes it easier to scale horizontally and reuse containers. For instance, a web application stack might consist of three separate containers, each with its own unique image, to manage the web application, database, and an in-memory cache in a decoupled manner.

Limiting each container to one process is a good rule of thumb, but it's not a hard and fast rule. For example, not only can containers be spawned with an init process, some programs might spawn additional processes of their own accord. For instance, Celeryopen_in_new can spawn multiple worker processes, and Apacheopen_in_new can create one process per request.

Use your best judgment to keep containers as clean and modular as possible. If containers depend on each other, you can use Docker container networks to ensure that these containers can communicate.

Sort multi-line arguments

Whenever possible, sort multi-line arguments alphanumerically to make maintenance easier. This helps to 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 imageopen_in_new:

RUN apt-get update && apt-get install -y \
  bzr \
  cvs \
  git \
  mercurial \
  subversion \
  && rm -rf /var/lib/apt/lists/*

Leverage build cache

When building an image, Docker steps through the instructions in your Dockerfile, executing each in the order specified. For each instruction, Docker checks whether it can reuse the instruction from the build cache.

The basic rules of build cache invalidation are as follows:

  • Starting with a parent image that's already in the cache, the next instruction is compared against all child images derived from that base image to see if one of them was built using the exact same instruction. If not, the cache is invalidated.

  • In most cases, simply comparing the instruction in the Dockerfile with one of the child images is sufficient. However, certain instructions require more examination and explanation.

  • For the ADD and COPY instructions, the modification time and size file metadata is used to determine whether cache is valid. During cache lookup, cache is invalidated if the file metadata has changed for any of the files involved.

  • Aside from the ADD and COPY commands, cache checking doesn't look at the files in the container to determine a cache match. For example, when processing a RUN apt-get -y update command the files updated in the container aren't examined to determine if a cache hit exists. In that case just the command string itself is used to find a match.

Once the cache is invalidated, all subsequent Dockerfile commands generate new images and the cache isn't used.

If your build contains several layers and you want to ensure the build cache is reusable, order the instructions from less frequently changed to more frequently changed where possible.

For more information about the Docker build cache and how to optimize your builds, see cache management.