fix(Makefile): ensure to use "latest" build of cedar:14

Heroku constantly updates the heroku/cedar:14 image as security patches are applied. This change enforces us to pull down the latest version of the image before building the image.

Author: Matthew Fisher

Makefile

@@ -21,7 +21,7 @@ bootstrap:
 	@echo Nothing to do.
 
 docker-build:
-	docker build --rm -t ${IMAGE} rootfs
+	docker build --pull --rm -t ${IMAGE} rootfs
 	docker tag ${IMAGE} ${MUTABLE_IMAGE}
 
 deploy: docker-build docker-push kube-pod