CI: Improve container image handling

This extends the pipeline definition to support forks to bring their own Docker registry. For now, our infrastructure will still use the registry.ostc-eu.org registry though, but anybody forking meta-openharmony to a GitLab server with Docker registry support will be able to simply use that.

To avoid increasing pipeline run-time caused by now (again) building container images, the container build jobs are changed to only be run when the container definitions are changed.

Related to eclipse/oniro-core/meta-openharmony#62