ref(Makefile): remove obsolete "docker tag -f" flag

This is an error with Docker 1.12.

Author: mboersma

Makefile

@@ -28,7 +28,7 @@ build: docker-build
 
 docker-build: check-docker
 	docker build --rm -t ${IMAGE} rootfs
-	docker tag -f ${IMAGE} ${MUTABLE_IMAGE}
+	docker tag ${IMAGE} ${MUTABLE_IMAGE}
 
 deploy: docker-build docker-push
 	sed 's#\(image:\) .*#\1 $(IMAGE)#' /tmp/deis-$(COMPONENT) | kubectl apply --validate=true -f -