Optimize build workflow

[sirewix]

• Why do we have separate build and release jobs? If there's no clear answer we should merge them into one
• After Add ghcr publish pipeline #20 we have a separate workflow for building image for ghcr. We should have one workflow that builds image and pushes to two registries (our nexus and ghcr). Or maybe we should stop pushing to nexus altogether?

[tlater-famedly]

#20 also does not use the famedly docker workflow; this seems to have mostly been because of lack of sensible documentation on that workflow making it hard to work with.

We should as part of this investigate how much of that workflow we actually need, and decide on one approach.

[sirewix]

This is somewhat important as it makes #15 twice as bad