Support alternative image host to avoid new Docker Hub limits

[ofek]

https://www.docker.com/blog/scaling-docker-to-serve-millions-more-developers-network-egress/

https://github.com/pypa/manylinux uses https://quay.io

[asottile]

easier is probably to use github's thing (ghcr) -- I've had some success with that here

I've also seen much better performance out of ghcr than quay, but that may be anecdotal

[antocuni]

I agree it would be a good idea to move away from docker hub. PRs are welcome ;)