Complete the example configuration for gcp cache

What does this MR do?

It completes the documentation for configuring Google Cloud Storage as cache-store using a JSON file for credentials with access to GCS. the

I had followed the instructions, but keep getting an error No URL provided, cache will not be downloaded from shared cache server. Instead a local version of cache will be extracted. I couldn't find any solution so I had to look under the hood of the helm chart to find that the JSON file wasn't mounted because the helm chart didn't know it was supposed to.

By adding the entry as seen in my code change, everything worked as expected.

To help the community, I figured an MR was the appropriate thing to do.

Related issues

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