Allow building experimenta etcd images

cluster/images/etcd/Makefile

@@ -18,6 +18,7 @@
 # 	[TAG=3.0.4] [REGISTRY=gcr.io/google_containers] [ARCH=amd64] [BASEIMAGE=busybox] make (build|push)
 
 TAG?=3.0.4
+REGISTRY_TAG?=$(TAG)
 ARCH?=amd64
 REGISTRY?=gcr.io/google_containers
 GOLANG_VERSION?=1.6.3
@@ -71,15 +72,15 @@ endif
 	cd $(TEMP_DIR) && sed -i.bak 's|BASEIMAGE|$(BASEIMAGE)|g' Dockerfile
 
 	# And build the image
-	docker build -t $(REGISTRY)/etcd-$(ARCH):$(TAG) $(TEMP_DIR)
+	docker build -t $(REGISTRY)/etcd-$(ARCH):$(REGISTRY_TAG) $(TEMP_DIR)
 
 push: build
-	gcloud docker push $(REGISTRY)/etcd-$(ARCH):$(TAG)
+	gcloud docker push $(REGISTRY)/etcd-$(ARCH):$(REGISTRY_TAG)
 
 ifeq ($(ARCH),amd64)
 	# Backward compatibility. TODO: deprecate this image tag
-	docker tag $(REGISTRY)/etcd-$(ARCH):$(TAG) $(REGISTRY)/etcd:$(TAG)
-	gcloud docker push $(REGISTRY)/etcd:$(TAG)
+	docker tag $(REGISTRY)/etcd-$(ARCH):$(REGISTRY_TAG) $(REGISTRY)/etcd:$(REGISTRY_TAG)
+	gcloud docker push $(REGISTRY)/etcd:$(REGISTRY_TAG)
 endif
 
 all: build