Fix gcloud docs for ssh-key mounts.
Fix gcloud docs for ssh-key mounts.

- The current `volumeMounts` config does not work, `mountPath` only accepts a folder and not a file unless you use "file projection" secrets. Also, in kubernetes YAML you can use octal `defaultMode: 0400` to be clearer about permissions (instead of `defaultMode: 254` which is required with kubernetes JSON).
- Also added a quick note about how to do container startup scripts.
buildkite/docs,buildkite/docs,buildkite/docs,buildkite/docs