modify 'release-build' make command to allow for additional build flags

Author: lisac

Makefile

@@ -306,7 +306,7 @@ endif
 release-build: ## Build release for $APP_NAME and tag it with current git hash
 	@:$(call check_defined, APP_NAME, the name of subdirectory of /infra that holds the application's infrastructure code)
 	cd $(APP_NAME) && $(MAKE) release-build \
-		OPTS="--tag $(IMAGE_NAME):latest --tag $(IMAGE_NAME):$(IMAGE_TAG)"
+		OPTS="--tag $(IMAGE_NAME):latest --tag $(IMAGE_NAME):$(IMAGE_TAG) $(OPTIONAL_BUILD_FLAGS)"
 
 release-publish: ## Publish release to $APP_NAME's build repository
 	@:$(call check_defined, APP_NAME, the name of subdirectory of /infra that holds the application's infrastructure code)