Expand Runner helm chart troubleshooting section

What does this MR do?

This MR is based on a couple of errors I faced when working on internal customer ticket:

• one is related to Switching to the Ubuntu-based gitlab-runner Docker image It expands the troubleshooting section with ideas how the errors can be fixed.

• another one is about the usage of authentication token when registering GitLab Runner helm chart.

Why was this MR needed?

To improve search-ability of the errors.