support custom registry image with `create cluster`

The --registry arg to ctlptl create cluster will ensure that ctlptl creates a registry for the cluster it creates.
ctlptl create registry has support for specifying a custom image. If a user creates a registry using that, and then tries to create a cluster using that registry, ctlptl will delete and recreate the registry with the default image (the test, desired.Image (hardcoded to empty string)).

I tried a bit to work around this by first creating the cluster, then deleting the registry, recreating it with a custom image name, and adding it to the docker network, but didn't manage to get it to work. Even if I could get it to work, that rather defeats the point of using ctlptl in the first place.

The two (compatible) main options I see are:

• add a --registry-image arg to ctlptl create cluster. This would presumably best solve the common case.
• add some way to tell ctlptl create cluster to skip registry creation and just connect to an existing registry. This would presumably leave a better escape hatch for any similar problem (for example).

Start with the registry behavior in pkg/registry/registry.go around the linked test and the cluster handling in pkg/cluster/cluster.go around desired.Image. Compare the existing custom-image support with the hardcoded empty image used by cluster creation. Resolve whether the change should add a registry-image option or support an existing registry, then add coverage showing the registry is not recreated with the default image.