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

Author: mboersma

.travis.yml

@@ -6,6 +6,8 @@ cache:
     - $HOME/.cache/pip
 services:
   - docker
+env:
+  - DOCKER_BUILD_FLAGS="--pull --no-cache"
 install:
   - make deps
 script:

Makefile

@@ -47,7 +47,7 @@ docker-test: docker-build-docs
 	${DEV_ENV_CMD} ${IMAGE} $(TEST_CMD)
 
 docker-build:
-	docker build --rm -t ${IMAGE} .
+	docker build ${DOCKER_BUILD_FLAGS} -t ${IMAGE} .
 
 docker-serve:
 	${DEV_ENV_CMD} ${IMAGE} $(MKDOCSSERVE)