[Enhancement] Divide docker pipeline so that it can also be used as a PR check

[kekonn]

Cut the docker pipeline up into multiple steps (build and push) so that we can do the build but skip the push step on PRs that touch anything in the Docker folder.

One thing we should definitely change is the docker metadata detection. Currently it only works if we run this from the main branch. Documentation for the action can be found here. Consider skipping this step entirely, since it's only useful when we push.