[CI] Build PR images on GHCR

[jlestel]

I modified the CI so that the PR images are build then pushed into the GitHub packages (GHCR).

This does not interfere with how current CI works with Docker Hub.

Functioning :

• When a PR is created or modified: the image is built on the GHCR from fork repo.
• When a branch is pushed: the image is built on the GHCR from main repo.
• When a PR is completed: the image is deleted in the fork repo. Additionally, untagged images are also deleted on this fork repo.

Once this PR is merged, each new / updated PR will trigger CI on the fork repo.

• Users will then see all these images on their fork. Eg: https://github.com/jlestel/teslamate/pkgs/container/teslamate

If necessary, anyone can then modify their docker-compose to use a PR image (eg: ghcr.io/jlestel/teslamate:master) for testing this PR instead of teslamate/teslamate:latest.

After the first run, packages are created in the user repo but images can't be pushed to this user repo.

Each user which want to build his images needs to :

1. Activate Actions in their repo
2. In the packages user page (eg: https://github.com/users/jlestel/packages/`), go to teslamate and teslamate/grafana settings and add the user/teslamate repository with read/write permissions.

[JakobLichterfeld]

lgtm, thanks again for the refactoring and the new workflow!

[DrMichael]

This is awesome!
Perhaps we should have a section for contributors in the documentation?

[JakobLichterfeld]

Perhaps we should have a section for contributors in the documentation?

Yeah, I thought the same thing, one step at a time.