Docker Development

How to keep your images small

Small images are faster to pull over the network and faster to load into memory when starting containers or services. There are a few rules of thumb to keep image size small:

• Start with an appropriate base image. For instance, if you need a JDK, consider basing your image on the official openjdk image, rather than starting with a generic ubuntu image and installing openjdk as part of the Dockerfile.

• Use multistage builds. For instance, you can use the maven image to build your Java application, then reset to the tomcat image and copy the Java artifacts into the correct location to deploy your app, all in the same Dockerfile. This means that your final image doesn’t include all of the libraries and dependencies pulled in by the build, but only the artifacts and the environment needed to run them.

  • If you need to use a version of Docker that does not include multistage builds, try to reduce the number of layers in your image by minimizing the number of separate RUN commands in your Dockerfile. You can do this by consolidating multiple commands into a single RUN line and using your shell’s mechanisms to combine them together. Consider the following two fragments. The first creates two layers in the image, while the second only creates one.

    RUN apt-get -y update
    RUN apt-get install -y python

    RUN apt-get -y update && apt-get install -y python

• If you have multiple images with a lot in common, consider creating your own base image with the shared components, and basing your unique images on that. Docker only needs to load the common layers once, and they are cached. This means that your derivative images use memory on the Docker host more efficiently and load more quickly.

• To keep your production image lean but allow for debugging, consider using the production image as the base image for the debug image. Additional testing or debugging tooling can be added on top of the production image.

• When building images, always tag them with useful tags which codify version information, intended destination (prod or test, for instance), stability, or other information that is useful when deploying the application in different environments. Do not rely on the automatically-created latest tag.

Where and how to persist application data

• Avoid storing application data in your container’s writable layer using storage drivers. This increases the size of your container and is less efficient from an I/O perspective than using volumes or bind mounts.

• Instead, store data using volumes.

• One case where it is appropriate to use bind mounts is during development, when you may want to mount your source directory or a binary you just built into your container. For production, use a volume instead, mounting it into the same location as you mounted a bind mount during development.

• For production, use secrets to store sensitive application data used by services, and use configs for non-sensitive data such as configuration files. If you currently use standalone containers, consider migrating to use single-replica services, so that you can take advantage of these service-only features.