use-buildset-registry: configure as a pull-through proxy

Docker has a curious behavior with respect to the registry-mirror
setting: it will only use the credentials for dockerhub itself.
Therefore, configure the buildset registry to be a pull-through
cache for dockerhub, but with no credentials, so that we can set
buildset registry credentials under the dockerhub url without
their actually being used for dockerhub.

Change-Id: I5daf93868d408d71cf2766fbe1bb17f2cfa8a3c7

roles/run-buildset-registry/tasks/main.yaml

@@ -73,6 +73,9 @@
       REGISTRY_AUTH: htpasswd
       REGISTRY_AUTH_HTPASSWD_PATH: /auth/htpasswd
       REGISTRY_AUTH_HTPASSWD_REALM: Registry Realm
+      REGISTRY_PROXY_REMOTEURL: https://registry-1.docker.io
+      REGISTRY_PROXY_USERNAME:
+      REGISTRY_PROXY_PASSWORD:
     volumes:
       - "{{ buildset_registry_root}}/data:/var/lib/registry"
       - "{{ buildset_registry_root}}/certs:/certs"