Enable multiarch build #1
Description
We currently auto-build c4-docker with Docker Hub's automated build system, which is only building and publishing amd64 images. I use a M1 Mac as my daily driver, which means I can't use these images as-is... which isn't a massive problem as I can build them locally but will make artefact evaluation for C4 work harder for people on M1 or other Arm processors.
I'm not sure of the best way to do a multiarch build yet, but we should set something up.