Raise Alpine Linux version to 3.21

[waja]

Release notes: https://wiki.alpinelinux.org/wiki/Release_Notes_for_Alpine_3.21.0

[hoellen]

@waja I need some time to test this properly. What tests have you already done?

[waja]

@hoellen I did build it locally and spin of the container. Maybe it might be a good idea to run the Github build pipeline also on PRs, which might give you a better indication if changes are okay before merging to master.

[waja]

Maybe it might be a good idea to run the Github build pipeline also on PRs, which might give you a better indication if changes are okay before merging to master.

I've prepared #97 to solve this