Run build-release within build image (googleforgames#2945)

Shifting the Cloud Build submission into the build image, so that it
utilises the credentials and configured Google Cloud project that is
set up at the beginning of the release process.

Co-authored-by: Robert Bailey <robertbailey@google.com>

build/includes/release.mk

@@ -96,5 +96,5 @@ do-release: $(ensure-build-image)
 	@echo "Now go make the $(RELEASE_VERSION) release on Github!"
 
 build-release:
-	cd $(agones_path) && gcloud builds submit . --substitutions _VERSION=$(base_version) --config=./build/release/cloudbuild.yaml $(ARGS)
-
+	docker run --rm $(common_mounts) -w $(workdir_path) $(build_tag) \
+		gcloud builds submit . --substitutions _VERSION=$(base_version) --config=./build/release/cloudbuild.yaml $(ARGS)