.github: build container images for arm64

[paulfantom]

As in title.

Spawned from https://github.com/prometheus-operator/kube-prometheus/pull/1720#pullrequestreview-937273045

I am not 100% sure this will work as I am not familiar with the toolchain used in the repository. However, according to https://github.com/docker/build-push-action/blob/master/docs/advanced/multi-platform.md this should do something :)

[metalmatze]

Oh yes! :rocket: Is this supposed to be Draft PR?

[paulfantom]

Is this supposed to be Draft PR?

Yes, I am still working on it ;)

[RiRa12621]

Run the setup step first maybe to see if the platforms are actually available --> setup-buildx-action

List of available platforms will be displayed and available through our setup-buildx action.

[paulfantom]

Yeah, I used draft PR exactly to catch such issues and figure out what next steps need to be done. I didn't want (or had time) to figure out the whole toolchain locally first.

[metalmatze]

Hey, a friendly ping and reminder this PR still exists. :relaxed:

[RiRa12621]

probably obsoleted now by https://github.com/pyrra-dev/pyrra/commit/b9570eec45762498d1d0fb3b9852bfff68aff47b ?

[metalmatze]

Indeed. Thanks for noting!