Adding simple cloudbuild file that builds and tags docker image

Author: kfswain

Diff for: cloudbuild.yaml

@@ -0,0 +1,15 @@
+steps:
+  - name: gcr.io/cloud-builders/docker
+    args:
+    - build
+    - --tag=us-central1-docker.pkg.dev/k8s-staging-images/llm-instance-gateway:$_GIT_TAG
+    - .
+substitutions:
+  _GIT_TAG: '12345'
+# this prevents errors if you don't use both _GIT_TAG
+# or any new substitutions added in the future.
+options:
+  substitution_option: ALLOW_LOOSE
+# this will push these images, or cause the build to fail if they weren't built.
+images:
+  - 'us-central1-docker.pkg.dev/k8s-staging-images/llm-instance-gateway:$_GIT_TAG'