Cherry pick 'jansmets-gitlab-705-task-runner-tmp' into '1-8-stable'

This change set is cherry-pick as an MR to ensure functionality within the 1.8.x line of releases.

Closes #1305 (closed)

Jansmets gitlab 705 task runner tmp

Closes #705

See merge request charts/gitlab!769

(cherry picked from commit 53777bce38de82e10b9d8502f508f5fa1e767868)

cebabe72 Persistent storage for task-runner tmp directory.
3080ae10 Updates to finish up change
03978d65 Update MR number in changelog entry
52b63706 Updating task-runner tmp changes
a01c52ce Fix path in link to backup docs