Build a multi-arch Docker image. Intended to close #215

[paul-snively]

This pull request revises the Dockerfile to use the xx Docker cross-compilation helpers and the Makefile to use Docker's buildx extension to build a multi-arch image.

[paul-snively]

@goccy, please forgive the imposition, but might I ask when this might be reviewed and, if acceptable, merged? I could change the package name in my fork and publish my own image, but my feeling is that would be undesirable compared to helping with the main project. But I also have colleagues at work who would benefit from its availability, so I wonder what a reasonable timeframe to expect is. Thank you so much for all your hard and good work on this!

[ohaibbq]

it seems silly for me to speak on goccy's behalf, but in my experience contributing over the last few months, they only have time to review and do releases every couple weeks, normally on the weekends. A sponsorship to the project also helps responsiveness!

We maintain a fork for our internal usages and open up PRs against both the upstream repository and our fork. If it is urgently blocking, i.e. you can't use the --platform linux/amd64 flag, I'd recommend doing the same.

The only thing you should need to change is the Github Images repository that the action publishes to in .github/workflows/build.yml