This is useful for testing container images from feature branches. However,
to avoid littering the artifact registry with endless feature branch
containers, we'll allow it as a manual task as required.

The manual job is marked as "allow_failure" as this is currently the only
way to allow the pipeline to show as successful if the (optional)
manual job is not triggered.
See the following issue for context
https://gitlab.com/gitlab-org/gitlab/-/issues/249524.