Add a "Pull a known Good Image" Smoke Test to Staging

Context

2023-05-12: Container registry pulls failing wi... (gitlab-com/gl-infra/production#14260 - closed) was caused by a bug that was difficult to detect via tests using images created by registry instances affected by the bug. In order to ensure continuity between old and new versions of the registry, we should have a smoke test running on the staging environment which pulls an image that's known to be well-formed and accessible to the CI job running the test. This way, we can ensure that new releases of registry don't slip out of compatibility with older ones.