Support make UPDATE_LATEST=false publish

When publishing pre-releases via the release script we do not want to
modify the `latest` tag in DockerHub - specifying UPDATE_LATEST=false
causes publish to push to $WEAVE_VERSION only. Omitting the flag
entirely retains the existing behaviour for backwards compatability.

Makefile

@@ -111,7 +111,9 @@ tests: $(COVER_EXE)
 $(PUBLISH): publish_%: $(IMAGES_UPTODATE)
 	$(SUDO) docker tag -f $(DOCKERHUB_USER)/$* $(DOCKERHUB_USER)/$*:$(WEAVE_VERSION)
 	$(SUDO) docker push   $(DOCKERHUB_USER)/$*:$(WEAVE_VERSION)
+ifneq ($(UPDATE_LATEST),false)
 	$(SUDO) docker push   $(DOCKERHUB_USER)/$*:latest
+endif
 
 publish: $(PUBLISH)