support custom registry image with `create cluster`

[landism]

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).

[landism]

For Chesterton's Fence purposes, Milas confirmed that this was simply an unconsidered use case.

[glennpratt]

I believe this is the same issue, but also defining these in a file causes the same issue. For me, that means the second approach would be the correct one.

This registry container will not have my environment variables. If I remove the registry: ctlptl-registry line, it will, but then it doesn't appear images from that registry work within the kind cluster.

apiVersion: ctlptl.dev/v1alpha1
kind: Registry
name: ctlptl-registry
port: 5005
image: docker.io/library/registry:3
env:
  - OTEL_TRACES_EXPORTER=none
  - REGISTRY_STORAGE={}
  - REGISTRY_STORAGE_S3_REGION=us-lax-1
  - REGISTRY_STORAGE_S3_SECURE=true
  # other items redacted
---
apiVersion: ctlptl.dev/v1alpha1
kind: Cluster
product: kind
registry: ctlptl-registry
kindV1Alpha4Cluster:
  name: tilt
  nodes:
    - role: control-plane
      image: kindest/node:v1.31.0