Allow use to mount volumes to runner pod with helm chart

Overview

When using a self-signed certificate with GitLab Runner you start getting x509: certificate signed by unknown authority when you try to register or do anything with GtiLab Runner. Usually, the user just hast to follow https://docs.gitlab.com/runner/configuration/tls-self-signed.html to add support for it. With the helm chart, you are not able to define volumes which means that we don't support self-signed certificates.

Proposal

• Allow user to attach volumes to runner manager containers
• Updated documentation in https://docs.gitlab.com/runner/configuration/tls-self-signed.html to show how to do it with

Original Report

Registration attempt 27 of 30 Runtime platform arch=amd64 os=linux pid=387 revision=gitlab-org/gitlab-runner@a987417a version=12.2.0 WARNING: Running in user-mode.
WARNING: The user-mode requires you to manually start builds processing: WARNING: $ gitlab-runner run
WARNING: Use sudo for system-mode:
WARNING: $ sudo gitlab-runner...

ERROR: Registering runner... failed runner=rATVoxxt status=462 PANIC: Failed to register this runner. Perhaps you are having network problems

Workaround

You can manually attach a volume but this is not suggested because if you updated the kubernetes deployment you can no longer update the helm chart.