Makefile: Fix override image tagging and add push override target

docker tag has no -f flag

Author: johanneswuerbach

Makefile

@@ -93,7 +93,7 @@ container:
 
 	docker build --pull -t $(PREFIX)/heapster-$(ARCH):$(VERSION) $(TEMP_DIR)
 ifneq ($(OVERRIDE_IMAGE_NAME),)
-	docker tag -f $(PREFIX)/heapster-$(ARCH):$(VERSION) $(OVERRIDE_IMAGE_NAME)
+	docker tag $(PREFIX)/heapster-$(ARCH):$(VERSION) $(OVERRIDE_IMAGE_NAME)
 endif
 
 ifndef DOCKER_IN_DOCKER
@@ -128,6 +128,9 @@ push-influxdb:
 push-grafana:
 	PREFIX=$(PREFIX) make -C grafana push
 
+push-override:
+	docker push $(OVERRIDE_IMAGE_NAME)
+
 gcr-login:
 ifeq ($(findstring gcr.io,$(PREFIX)),gcr.io)
 	@echo "If you are pushing to a gcr.io registry, you have to be logged in via 'docker login'; 'gcloud docker push' can't push manifest lists yet."