Add Qt 5.15 based Musl libc (Alpine Linux) CI image

Alpine Linux is a platform that uses Musl libc by default rather than glibc. This has some implications and it has already caused failing builds on this platform before on several repositories because they assume glibc. The issues are however not hard to solve and just need to be found in time, having a Musl-based CI image helps here.

I've based this on the existing suse-qt515 image, and if need be I can add a Qt 6.2 image as well. I'm not sure all required deps are being installed now. The image builds but I don't know how to test it properly in an actual CI setting.