Deploy to ghcr.io

[pbrkr]

This deployment process has been tested on my GitHub account using an extra commit to change the destination repo and not push to Docker Hub: https://github.com/pbrkr/yocto-dockerfiles/commit/74021a4badb7d9b447d4a577901da7bc75c54b98

The action results are all good there: https://github.com/pbrkr/yocto-dockerfiles/actions/runs/3928894387

And the containers are visible within my user account: https://github.com/pbrkr/yocto-dockerfiles/pkgs/container/yocto/versions

This should work when merged to the main yocto-dockerfiles repo but deployments are one of those things that's not possible to perfectly test before merging. Please ping me on IRC before merging this so we can check the results and I can jump in and fix things if there is an issue.

Once this deployment process is confirmed working for yocto-dockerfiles I'll move on to looking at the other repositories.

[moto-timo]

We'll probably want to document that the containers can then be pulled from ghcr.io/crops/yocto:ubuntu-20.04-base for example, but that is a separate issue.

Logical variable names and following the existing paradigm, so LGTM.