playground: push image in trigger before deploying

Removes post-build trigger step of pushing image, and adds a push step
step before AppEngine deployment. This ensures that the image we just
built is pushed before asking AppEngine to deploy it.

This is documented at:
https://cloud.google.com/cloud-build/docs/configuring-builds/build-test-deploy-artifacts

Cloud Build is authorized to push images to gcr.io. This change was
tested by submitting a build with this configuration without the
deployment step.

Updates golang/go#32606

Change-Id: I4e7cc242566378e4ca6d52244fb81bf410e79314
Reviewed-on: https://go-review.googlesource.com/c/playground/+/185339
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
diff --git a/cloudbuild_trigger.json b/cloudbuild_trigger.json
index d37d803..620d320 100644
--- a/cloudbuild_trigger.json
+++ b/cloudbuild_trigger.json
@@ -45,6 +45,13 @@
         ]
       },
       {
+        "name": "gcr.io/cloud-builders/docker",
+        "args": [
+          "push",
+          "gcr.io/$PROJECT_ID/playground"
+        ]
+      },
+      {
         "dir": "playground",
         "name": "gcr.io/cloud-builders/gcloud",
         "args": [
@@ -56,9 +63,6 @@
         ]
       }
     ],
-    "timeout": "1800s",
-    "images": [
-      "gcr.io/$PROJECT_ID/playground"
-    ]
+    "timeout": "1800s"
   }
 }