Allow configuring docker image when running make

Signed-off-by: Mikkel Oscar Lyderik Larsen <mikkel.larsen@zalando.de>

Makefile

@@ -11,6 +11,7 @@
 # ANY KIND, either express or implied. See the License for the specific
 # language governing permissions and limitations under the License.
 #
+IMAGE   ?= amazon/amazon-k8s-cni
 
 .PHONY: build-linux clean docker docker-build lint unit-test vet
 
@@ -31,8 +32,8 @@ docker-build:
 
 # Build docker image
 docker: docker-build
-	@docker build -f scripts/dockerfiles/Dockerfile.release -t "amazon/amazon-k8s-cni:$(VERSION)" .
-	@echo "Built Docker image \"amazon/amazon-k8s-cni:$(VERSION)\""
+	@docker build -f scripts/dockerfiles/Dockerfile.release -t "$(IMAGE):$(VERSION)" .
+	@echo "Built Docker image \"$(IMAGE):$(VERSION)\""
 
 # unit-test
 unit-test: