This PR changes the 'build-and-push-to-registry' workflow to only run on
the Pine64 repository, which prevents image pushing errors occurring.
By doing this, we ensure the container images only come from the Pine64
OpenPineBuds repository.
This commit adds support for a container build workflow, that, upon
pushes to the repo, but *not* pull requests, builds a container image at
one at a time, and pushes to GitHub Registry.
Dom Rodriguez