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Use multi-stage builds

Multi-stage builds are a new feature requiring Docker 17.05 or higher on the daemon and client.

Multistage builds are useful to anyone who has struggled to optimize Dockerfiles while keeping them easy to read and maintain.

Acknowledgment: Special thanks to Alex Ellis for granting permission to use his blog post Builder pattern vs. Multi-stage builds in Docker as the basis of the examples below.

Before multi-stage builds

One of the most challenging things about building images is keeping the image size down. Each instruction in the Dockerfile adds a layer to the image, and you need to remember to clean up any artifacts you don’t need before moving on to the next layer. To write a really efficient Dockerfile, you have traditionally needed to employ shell tricks and other logic to keep the layers as small as possible and to ensure that each layer has the artifacts it needs from the previous layer and nothing else.

It was actually very common to have one Dockerfile to use for development (which contained everything needed to build your application), and a slimmed-down one to use for production, which only contained your application and exactly what was needed to run it. This has been referred to as the “builder pattern”. Maintaining two Dockerfiles is not ideal.

Here’s an example of a Dockerfile.build and Dockerfile which adhere to the builder pattern above:

Dockerfile.build: