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

Author: mboersma

.travis.yml

@@ -7,6 +7,8 @@ cache:
     - vendor
 services:
   - docker
+env:
+  - DOCKER_BUILD_FLAGS="--pull --no-cache"
 install:
   - make bootstrap
 script:

Makefile

@@ -49,5 +49,5 @@ test-style:
 # For cases where we're building from local
 # We also alter the RC file to set the image name.
 docker-build: build
-	docker build --rm -t ${IMAGE} rootfs
+	docker build ${DOCKER_BUILD_FLAGS} -t ${IMAGE} rootfs
 	docker tag ${IMAGE} ${MUTABLE_IMAGE}