Update Helm chart troubleshooting for missing secrets
What does this MR do?
Explains how to handle volume mount problems with a Runner Helm installation.
Why was this MR needed?
There was an attempt to solve this in a nicer way with gitlab-org/charts/gitlab-runner!327 (closed), but unfortunately it would break existing installations. Adding resolution to troubleshooting should hopefully guide users if they get stuck with this.
What's the best way to test this MR?
N/A