Use the regionless mirror alias

cluster/gce/util.sh

@@ -243,12 +243,12 @@ function set-preferred-region() {
   else
     KUBE_ADDON_REGISTRY="gcr.io/google_containers"
   fi
-
-  if [[ "${ENABLE_DOCKER_REGISTRY_CACHE:-}" == "true" ]]; then
-    DOCKER_REGISTRY_MIRROR_URL="https://${preferred}-mirror.gcr.io"
-  fi
 }
 
+if [[ "${ENABLE_DOCKER_REGISTRY_CACHE:-}" == "true" ]]; then
+  DOCKER_REGISTRY_MIRROR_URL="https://mirror.gcr.io"
+fi
+
 # Take the local tar files and upload them to Google Storage.  They will then be
 # downloaded by the master as part of the start up script for the master.
 #