feat: add GHCR support for container registry

[gabor-boros]

This PR adds GHCR support for the container registry, so GitHub can be used over DockerHub or AWS ECR. Also, the PR sets the generated images as job outputs, so the images can be referenced in later steps if needed.

The later one is particularly useful when someone wants to build a pipeline where service build jobs are running parallel and the config.yml file should be updated at once.

[felipemontoya]

@magajh @DeimerM could you please take a look at this? I'll add it in the next sprint

[gabor-boros]

@magajh / @DeimerM is there anything I could help with to move this forward?

[magajh]

Thank you for this contribution!

Regarding the “Show image name for service” part, we’re planning to apply a fix in the upcoming sprint so that the image tag push doesn’t fail (in the dynamic tag case) when multiple builds are running or when there’s a push to the repository in the middle of a build. That said, the change you’re proposing here still feels very useful as a safeguard for edge cases where updating the tag in the repo fails for some reason and the user still needs to see which tag was generated

[gabor-boros]

@magajh sorry, this was a "bit" delayed. I removed the image tag outputs as it is not used anymore and it makes no sense to contribute something that used by noone. Instead, I focused on the documentation and added a new section to the readme listing and describing the current registries.