include sandbox variables

.gcp/release-docker.yaml

@@ -22,6 +22,8 @@ steps:
     id: 'Build sandbox Docker image'
     entrypoint: 'npm'
     args: ['run', 'build:sandbox:fast']
+    env:
+      - 'GEMINI_SANDBOX=$_CONTAINER_TOOL'
 
   # Step 7: Publish sandbox container image
   - name: 'us-west1-docker.pkg.dev/gemini-code-dev/gemini-code-containers/gemini-code-builder'
@@ -29,8 +31,13 @@ steps:
     entrypoint: 'npm'
     args: ['run', 'publish:sandbox']
     env:
+      - 'GEMINI_SANDBOX=$_CONTAINER_TOOL'
       - 'GEMINI_SANDBOX_IMAGE_TAG=$SHORT_SHA'
 
 options:
   defaultLogsBucketBehavior: REGIONAL_USER_OWNED_BUCKET
-  dynamicSubstitutions: true
+  dynamicSubstitutions: true
+
+substitutions:
+  _OFFICIAL_RELEASE: 'false'
+  _CONTAINER_TOOL: 'docker'