Makefile+github: add make fmt-check to check for formatting

Makefile

@@ -258,6 +258,10 @@ fmt: $(GOIMPORTS_BIN)
 	@$(call print, "Formatting source.")
 	gofmt -l -w -s $(GOFILES_NOVENDOR)
 
+fmt-check: fmt
+	@$(call print, "Checking fmt results.")
+	if test -n "$$(git status --porcelain)"; then echo "code not formatted correctly, please run `make fmt` again!"; git status; git diff; exit 1; fi
+
 lint: docker-tools
 	@$(call print, "Linting source.")
 	$(DOCKER_TOOLS) golangci-lint run -v $(LINT_WORKERS)