change location of docker images to ghcr.io

[flavorjones]

See context from #71 and #87

This PR moves from dockerhub larskanis account to the github registry for the rake-compiler org. It keeps support for the DOCKERHUB_USER environment variable but also supports an env var with a more appropriate name, DOCKER_REGISTRY.

@larskanis Would love your feedback on this!

[flavorjones]

@larskanis I think I'd like to use a single container repository for all the images. Would you be open to changing the container name and tag format from:

#{REGISTRY}/rake-compiler-dock-#{RUBYVM}#{PLATFORM_POSTFIX}:#{VERSION}

to:

#{REGISTRY}/rake-compiler-dock-image:#{VERSION}-#{RUBYVM}#{PLATFORM_POSTFIX}

So, for example:

• ghcr.io/ghcr.io/rake-compiler/rake-compiler-dock-image:1.3.0-mri-x86_64-linux
• ghcr.io/ghcr.io/rake-compiler/rake-compiler-dock-image:1.3.0-jruby
• ghcr.io/ghcr.io/rake-compiler/rake-compiler-dock-image:snapshot-mri-arm64-darwin

The main advantage is that we could use a single image repository, ghcr.io/rake-compiler/rake-compiler-dock-images (rather than a separate repository per platform), which would simplify administration. This is also similar to what many other projects do: use the label as an indicator of both version and "flavor" (e.g., ruby/ruby:3.2-alpine).

This would also still work with the idea of shipping different host architectures, see #79.

[larskanis]

Yes, there are so many images now, that we should move the platform into the tag part. I already thought about it and when we touch the logic now, it's a good chance to change it now.

[flavorjones]

OK, I'm traveling a bit today but will get to this in the next day or so.

[flavorjones]

Running a Nokogiri pipeline using this repo and the snapshot images here: https://github.com/sparklemotion/nokogiri/pull/2756

Edit: those CI jobs went green!