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

Allows CI to pass in "--pull --no-cache" to avoid
surprises when building Docker images.

Author: mboersma

Jenkinsfile

@@ -84,7 +84,7 @@ node(linux) {
 
 				sh """
 					docker login -e="${quayEmail}" -u="${quayUsername}" -p="\${QUAY_PASSWORD}" quay.io
-					docker build -t ${test_image} .
+					docker build \${DOCKER_BUILD_FLAGS} -t ${test_image} .
 					docker push ${test_image}
 				"""
 

Makefile

@@ -68,7 +68,7 @@ test: build-test-image
 	docker run --rm ${IMAGE} test
 
 build-test-image:
-	docker build -t ${IMAGE} .
+	docker build ${DOCKER_BUILD_FLAGS} -t ${IMAGE} .
 
 push-test-image: build-test-image
 	docker push ${IMAGE}