Build and push multi-arch Docker images on ghcr.io

[sandhose]

This does two things:

• enhance the Dockerfile to be multi-arch capable (also allows it to run as non-root)
• build and push a Docker image on ghcr.io/matrix-org/rageshake via GitHub Actions

There are a few important things to note about the changes in the Dockerfile:

• the runtime image is based on an image from the distroless project
• this allows for the container to run as non-root (there is a /etc/passwd entry for that)
• the image can be cross-compiled (e.g. building the amd64 image from an arm64 host and vice-versa) with no qemu emulation involved. This however requires a buildkit-enabled Docker (which is the default on Docker Desktop), e.g. export DOCKER_BUILDKIT=1 on Linux
• I tested the image both on my M1 MacBook (arm64) and my x86_64 desktop
• I switched the /rageshake binary to be the entrypoint instead of the command. This allows to run the image like docker run ghcr.io/matrix-org/rageshake -help instead of docker run ghcr.io/matrix-org/rageshake /rageshake -help

[sandhose]

It happened: I needed to pop a shell within the container to debug something, but the image does not include one. I added a debug variant of the image (based on the debug variant of the distroless image), and to avoid building the image from scratch twice, I switched to Docker Buildx bake files, which allows specifying multiple targets at once, building only one build graph, which in the end avoids compiling the Go binary twice for both variants.

I have a similar setup over at https://github.com/matrix-org/matrix-authentication-service