Allow overriding the tag and registry
The same way it is possible in controllers/nginx/Makefile
# limitations under the License.
# 0.0.0 shouldn't clobber any released builds
-TAG = 0.25
-REGISTRY = gcr.io/google_containers
+TAG ?= 0.25
+REGISTRY ?= gcr.io/google_containers
ARCH ?= $(shell go env GOARCH)
ALL_ARCH = amd64 arm arm64 ppc64le
