Feature: Use the docker/metadata-action to set tags and labels for image

[stumpylog]

This PR updates the workflows to use docker/metadata-action to set both image tags and labels.

For a PR, this sets a tag such as pr-1 and a bunch of useful labels

For a release, this does update slightly, to produce both a :latest, a :version and a :major.minor, which looks like this with basically the same labels

Some other updates:

• This should allow forks to build and publish an image if they want
• Instead of needing a Personal Access Token, ${{ secrets.GITHUB_TOKEN }} will have permissions already
• Uses the ${{ github.actor }} instead of hard coding that, again for better fork support
• Disabled example of pushing to Docker Hub

[jordan-dalby]

@stumpylog
Nice! Are you aware of a way we can get PRs pushing to the registry? I use them for testing on my server and for users trying out fixes.

[stumpylog]

Not a problem, that should do it. Don't want to interrupt your workflow.

Hmm, ok, give me a bit

[jordan-dalby]

Nice work!