Makefile clarification for staging image, error if not set

Author: davidz627

Makefile

@@ -12,6 +12,9 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+# Args:
+# GCE_PD_CSI_STAGING_IMAGE: Staging image repository
+
 STAGINGIMAGE=${GCE_PD_CSI_STAGING_IMAGE}
 STAGINGVERSION=latest
 
@@ -25,6 +28,9 @@ gce-pd-driver:
 	go build -ldflags "-X main.vendorVersion=${STAGINGVERSION}" -o bin/gce-pd-csi-driver ./cmd/
 
 build-container:
+	ifndef GCE_PD_CSI_STAGING_IMAGE
+    	$(error "Must set environment variable GCE_PD_CSI_STAGING_IMAGE to staging image repository")
+    endif
 	docker build --build-arg TAG=$(STAGINGVERSION) -t $(STAGINGIMAGE):$(STAGINGVERSION) .
 
 push-container: build-container