Fix builds by switching to native arm64 runners

[lhoBas]

Enjoyed tinkering around with the GitHub Action so decided to take a stab at fixing #20 myself (with help from Copilot through Cline, to be fair)

[moritzheiber]

So the main challenge for this is (and the also the reason I haven't had a chance to try it yet): you need to build the images on different runners (because of the diverging architectures), but then "unify" them in at a later stage to push a proper multi-arch image instead of individual images (otherwise you're overwriting the image tag with just a single architecture, and not even deterministically, but rather the latest job that finishes snatches the tag). I don't believe this PR "solves" this challenge (yet)

[lhoBas]

Ah, I see - I'll push a manifest merge along the lines of docker/build-push-action#671 (comment) tomorrow 👌

[lhoBas]

This now handles creating a clean multiarch image for each image/version combo. Don't love how the manifest sha's are being passed across the steps, but apparently matrix output gathering is not very mature in GitHub Actions yet. No 'intermediary' tags are pushed to the GitHub Packages entry with this approach (example)

[moritzheiber]

I think this looks good enough, thank you!