feat: support dynamic image tags (#345)

Author: shruti-mantri

cloudbuild.yaml

@@ -15,22 +15,29 @@
 # Default cloudbuild.yaml for Cloud Shell custom images.
 # This file is used by the Cloud Build trigger that builds this image.
 
+substitutions:
+  _IMAGE_TAG: '${COMMIT_SHA}'
+
 steps:
   - name: "gcr.io/cloud-builders/docker"
     args:
       [
         "build",
         "-t",
-        "gcr.io/$PROJECT_ID/button",
+        "gcr.io/$PROJECT_ID/button:latest",
         "-t",
-        "gcr.io/$PROJECT_ID/button:public-image-${COMMIT_SHA}",
+        "gcr.io/$PROJECT_ID/button:public-image-${_IMAGE_TAG}",
         ".",
       ]
   - name: "gcr.io/cloudshell-images/custom-image-validation"
     args: ["image_test.py", "--image", "gcr.io/$PROJECT_ID/button"]
+
 images:
-  - "gcr.io/$PROJECT_ID/button"
-  - "gcr.io/$PROJECT_ID/button:public-image-${COMMIT_SHA}"
+  - "gcr.io/$PROJECT_ID/button:latest"
+  - "gcr.io/$PROJECT_ID/button:public-image-${_IMAGE_TAG}"
+
 timeout: "3600s"
+
 options:
   machineType: "N1_HIGHCPU_8"
+  dynamicSubstitutions: true