Create musllinux images

[lkollar]

We should probably create a separate repo for this, but opening the issue here for now.

As PEP 656 has been accepted, and work has started on adding musllinux support in auditwheel, we should also create Docker images for building wheels for musl.

I think the obvious choice to base these images on is Alpine as it is by far the most popular musl distro. We should also decide on the platform tag and musl version we want to start with. Alpine 3.12 is the oldest, still supported version so that sounds like a good candidate. It ships musl 1.1.24.