Add build cache for building play image (#1797)

**What this PR does / why we need it**:

**Which issue(s) this PR fixes**:

Fixes #

**Does this PR introduce a user-facing change?**:
<!--
If no, just write "NONE" in the release-note block below.
-->
```release-note
NONE
```

This PR was merged by Kapetanios.

.kapetanios/postsubmit.yaml

@@ -114,8 +114,11 @@ spec:
         description: Build static files
         runner: gcr.io/pipecd/runner:1.0.0
         commands:
-          - bazelisk build //pkg/app/web:public_files --action_env=ENABLE_MOCK=true
+          - bazelisk --output_base=/workspace/bazel_out build --config=ci //pkg/app/web:public_files --action_env=ENABLE_MOCK=true
           - cp -rf $(bazelisk info bazel-bin)/pkg/app/web/public_files ./play/public
+        secrets:
+          - name: bazel_cache_service_account
+            type: PROJECT
       - name: docker-build
         description: Build docker image
         runner: gcr.io/cloud-builders/docker