Multi arch builds

As discussed here: https://github.com/legendu-net/docker-rustpython/pull/9#issuecomment-1367722385

I use it on my image: https://github.com/sudo-bot/docker-phpldapadmin/blob/370c6f8b198f0e233e488c577d7d6a43bf24bd0e/docker/Dockerfile#L15