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

Author: mboersma

.travis.yml

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

Makefile

@@ -46,7 +46,7 @@ build: check-docker
 	${DEV_ENV_CMD} make binary-build
 
 docker-build: build check-docker
-	docker build --rm -t ${IMAGE} rootfs
+	docker build ${DOCKER_BUILD_FLAGS} -t ${IMAGE} rootfs
 	docker tag ${IMAGE} ${MUTABLE_IMAGE}
 
 # Builds the binary-- this should only be executed within the