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

Author: mboersma

.travis.yml

@@ -9,6 +9,7 @@ cache:
     - vendor
 env:
   DEV_REGISTRY=quay.io
+  DOCKER_BUILD_FLAGS="--pull --no-cache"
 services:
   - docker
 sudo: required

Makefile

@@ -45,7 +45,7 @@ update-changelog:
 	  | cat - CHANGELOG.md > tmp && mv tmp CHANGELOG.md
 
 docker-build: build-binary
-	docker build --rm -t ${IMAGE} rootfs
+	docker build ${DOCKER_BUILD_FLAGS} -t ${IMAGE} rootfs
 	docker tag ${IMAGE} ${MUTABLE_IMAGE}
 
 .PHONY: all docker-build test