docker: Distributed multiarch

This would be very useful since you can just split each platform to a dedicated runner

https://docs.docker.com/build/ci/github-actions/multi-platform/#distribute-build-across-multiple-runners

Start with the linked Docker documentation on distributing multi-platform builds across multiple runners, then inspect this repository's shared GitHub Actions workflow definitions. Determine how platform-specific runners and the final multiarch image would be coordinated; done means the workflow builds each platform on a dedicated runner and produces the intended multiarch result.

Written by the indexing model from the issue text.