ci: build-oci-image: fix logic for detecting changes to .docker in build-oci-image
ci: build-oci-image: use registry url variable, not hardcoded url
ci: build-oci-image: change changes_detected var to build image
ci: build-oci-image: output an image tag for other jobs to use
ci: build-oci-image: cache the docker image build
This wont always speed things up, but occasionally it will speed things
up by a lot
