Build containers only once and also do it for forks

[sharkwouter]

This PR changes the following:

• Always build the containers only once (at the moment they are build twice for commits to master)
• If this is the master branch, push containers to the repository's container registry first. This makes it so forks can also get container images.

What stays the same:

• If this is the master branch and the XboxDev repo specifically, also push containers to the group's registry with the tags which we're currently used to (nxdk:git-..., nxdk-debug:latest, etc).
• If this is the master branch and the XboxDev repo specifically, also push containers to the docker hub with the tags which we're currently used to (nxdk:git-..., nxdk-debug:latest, etc).

This is a first step towards a multi-step build using docker. I'd like to use that to be able to build packages which can be installed using a package manager I've been working on. I think this specific part is useful regardless of if that's the way you guys will want to go in the end, though.

[thrimbor]

I've read through all the changes as carefully as I could and they look good to me. I do think the commits should get consolidated before we merge though to get rid of fix-up commits. Squashing it all into a single commit would be fine imho.

[sharkwouter]

Thanks for the review @thrimbor! I've squished the commits into a single one now. Can you take another look? I've not made any other changes.