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

Author: mboersma

.travis.yml

@@ -10,6 +10,7 @@ services:
 env:
   # HACK(bacongobbler): make travis tests work
   - DEIS_REGISTRY=travis-ci
+  - DOCKER_BUILD_FLAGS="--pull --no-cache"
 install:
   - make docker-build
 script:

Makefile

@@ -35,7 +35,7 @@ build: check-docker
 # For cases where we're building from local
 # We also alter the RC file to set the image name.
 docker-build: check-docker build
-	docker build --rm -t ${IMAGE} rootfs
+	docker build ${DOCKER_BUILD_FLAGS} -t ${IMAGE} rootfs
 	docker tag ${IMAGE} ${MUTABLE_IMAGE}
 
 build-binary: