Add env property on Registry

[MalibuKoKo]

Acutally it's not possible to manage env vars of container registry.

Please let'me override registry configuration : https://docs.docker.com/registry/configuration/

A use case, inspirated by following post : https://maelvls.dev/docker-proxy-registry-kind/ is to declare a registry as a docker proxy for kind.

---

Create network, proxy and cluster; attach proxy to the network :

export KIND_EXPERIMENTAL_DOCKER_NETWORK=cluster
docker network create --scope local --driver bridge --subnet 10.20.0.0/16 --gateway 10.20.0.1 $KIND_EXPERIMENTAL_DOCKER_NETWORK
ctlptl apply -f /dev/stdin <<EOF                                                                                                                          
apiVersion: ctlptl.dev/v1alpha1
kind: Registry
name: proxy-docker-hub
env:
- REGISTRY_PROXY_REMOTEURL=https://registry-1.docker.io
---
apiVersion: ctlptl.dev/v1alpha1
kind: Cluster
product: kind
kubernetesVersion: "1.26"
kindV1Alpha4Cluster:
  name: $KIND_EXPERIMENTAL_DOCKER_NETWORK
  featureGates:
    PodSecurity: true
  containerdConfigPatches:
  - |-
    [plugins."io.containerd.grpc.v1.cri".registry.mirrors."docker.io"]
      endpoint = ["http://proxy-docker-hub:5000"]
  nodes:
  -   role: control-plane
EOF
docker network connect $KIND_EXPERIMENTAL_DOCKER_NETWORK proxy-docker-hub

---

Pull an image inside cluster :

docker exec -it $KIND_EXPERIMENTAL_DOCKER_NETWORK-control-plane crictl pull docker.io/library/alpine:3.9.6

---

Check proxy's logs :

docker logs -f proxy-docker-hub

---

[nicks]

LGTM!