Support for multiarch images

[jonkerj]

What this PR does / why we need it:

This PR alters the Dockerfiles for both regular and OpenShift in such a way multiarch images are getting built when using buildkit. Also, the GH workflow actually building the images is prepared for that.

Also, I did a bit of cleaning up to ease testing.

Which issue(s) this PR fixes

Fixes #226

Special notes for your reviewer (optional):

1. I've disabled CGO in the OpenShift dockerfile, as it would require a cross compiler (gcc). I've made the resulting binary slightly bigger, but the build setup is much cleaner this way.
2. I tested the Dockerfiles by building them locally (docker buildx build --platform linux/amd64,linux/arm64 --file <dockerfile>)
3. I've adjusted the workflow to also build/push in forks, which led to proper MA images in our fork's GHCR. One test is currently failing (send to coveralls 500's), but I believe it's unrelated to my changes
4. The tags for -amd64/-arm64 are gone, as the image contains both
5. Openshift's workflow/dockerfile requires qemu (the regular doesn't), because of the RUN mkdir in that dockerfile
6. I've performed LCM on most of the used GH actions
7. I had to patch the e2e, as it needs to use buildkit now.

[jonkerj]

I think I'm done now, it's ready for review

[jonkerj]

Anything else I can do, @maiqueb?

[jonkerj]

It is kind of annoying that this PR has partly been superseded by #329 and rebasing my PR requires a total rewrite now...

I'm willing to rebase/refresh the part that is still left (combine Dockerfile/Dockerfile.arm64 and eliminate QEMU by having go cross compile), but only if someone is actually going to take a look at it (@maiqueb or @dougbtv). Please let me know if this is the case.

[maiqueb]

I apologize for that.

I can take a look at it if you eliminate the QEMU dependency from this repo.

[dougbtv]

Thanks for the rebase, and apologies for the lack of coordination. Next time, feel free to join the Multus maintainer's call (info @ https://github.com/k8snetworkplumbingwg/community ) if you're relying on an update that isn't merged or isn't getting traction.