Native Multi-arch Docker Builds in Github Actions

As we wait for Github's ARM-based Linux runners to be available, multi-arch Docker builds remain annoying to deal with if you want to avoid using QEMU. Fortunately, this becomes easy if you have an ARM-based machine machine at your disposal.

In this article, we will use this machine as a remote builder controlled by Docker Buildx on another machine (a Github Actions runner) to build our multi-arch images completely natively.

What's especially nice about this approach is that we don't need to deal with manifest files, which would be necessary if we were to use a separate arm-based runner to build the arm images in parallel with the x86 images.

#Accessing the ARM machine

We'll test that we can access the machine via docker first. I like using SSH configs, so here's mine:

We use the format ssh://<HOST> to access the machine via Docker, or you can use ssh://<USER>@<HOST> if you're not using a config file.

#Sample Github Actions Workflow

Now that the docker connection is working, here's a sample Github Actions workflow that does the following:

1. Sets up an SSH config file
2. Connects to the ARM machine via Docker Buildx as a remote builder
3. Builds an pushes the image to GHCR

You can check out joshuanianji/arm-docker-images for some more examples.

#Caching Pitfalls

I use this approach in my idris-2-docker repo, but have struggled with proper caching. The issue seems to be that both caches are being saved in the Github Actions cache and none on the remote builder, so we need a full build on the remote builder every time.

Still, that hypothesis makes no sense in light of this Stack Overflow answer. Perhaps this is an issue with my workflow setup, so I guess I need to do some more digging.