Add harbor to our integration testing suite?

### What is the problem you're trying to solve

Right now, we rely solely on distribution registry (and cesanta/docker_auth) for login/push/pull testing.

It is a possibility that Harbor has some specific behaviors diverging from mainline registry (eg, looking at https://github.com/containerd/nerdctl/issues?q=is%3Aissue+is%3Aopen+harbor )

### Describe the solution you'd like

Suggestion is to add https://hub.docker.com/r/bitnami/harbor-registry as one of our option, and tests against it as well.

If this does not uncover any issue at all, then we can consider disabling it.

### Additional context

_No response_