Stop publishing tags for PRs (#74)

Makefile

@@ -87,19 +87,24 @@ endif
 endif
 ifndef TAG
 override TAG=$(SHA)
+override DO_NOT_PUBLISH=1
 endif
 
 docker-build: set-tag
     #NOTE: Assumes binary has already been built (admiral)
 	docker build -t $(IMAGE):$(TAG) -f ./admiral/docker/Dockerfile.admiral .
 
 docker-publish:
+ifndef DO_NOT_PUBLISH
 	echo "$(DOCKER_PASS)" | docker login -u $(DOCKER_USER) --password-stdin
+endif
 ifeq ($(TAG),)
 	echo "This is not a Tag/Release, skipping docker publish"
 else
+ifndef DO_NOT_PUBLISH
 	docker push $(IMAGE):$(TAG)
 endif
+endif
 #no tag set and its master branch, in this case publish `latest` tag
 ifeq ($(TAG),)
 ifeq ($(BRANCH),master)