feat(Makefile): set docker build flags via environment variable

Author: mboersma

Makefile

@@ -12,7 +12,7 @@ uninstall: kube-delete
 upgrade: kube-update
 
 docker-build:
-	docker build -t ${IMAGE} rootfs
+	docker build ${DOCKER_BUILD_FLAGS} -t ${IMAGE} rootfs
 	docker tag ${IMAGE} ${MUTABLE_IMAGE}
 
 kube-delete:

versioning.mk

@@ -19,4 +19,4 @@ docker-immutable-push:
 
 .PHONY: docker-mutable-push
 docker-mutable-push:
-	docker push ${MUTABLE_IMAGE}
+	docker push ${MUTABLE_IMAGE}