Updates Makefile to use generic Docker registry

examples/wordpress/images/nginx/Makefile

@@ -1,6 +1,7 @@
 .PHONY: all build push clean
 
-PREFIX = gcr.io/$(PROJECT)
+DOCKER_REGISTRY = gcr.io
+PREFIX = $(DOCKER_REGISTRY)/$(PROJECT)
 IMAGE = nginx
 TAG = latest
 
@@ -12,7 +13,11 @@ build:
 	docker build -t $(PREFIX)/$(IMAGE):$(TAG) $(DIR)
 
 push: build
+ifeq ($(DOCKER_REGISTRY),gcr.io)
 	gcloud docker push $(PREFIX)/$(IMAGE):$(TAG)
+else
+	docker push $(PREFIX)/$(IMAGE):$(TAG)
+endif
 
 clean:
 	docker rmi $(PREFIX)/$(IMAGE):$(TAG)