ingress-nginx: Add coredns configuration to resolve registry internally

Add some custom CoreDNS configuration so that registry.cloud.releng.team resolves to the cluster IP of the ingress-nginx-controller from within the cluster. This will allow processes within the cluster to reach the registry using the nginx based ingress instead of the service directly, making use of its TLS termination, JWT auth (future work), and caching.

Note that following this change, we'll need to tweak the ingress-nginx config that limits request methods to the registry to only apply to external ingress, not requests from within the cluster.