Add docs and an example for users bringing their own images
This ties in with our work on hardened container images, but its currently not well documented how users who build their own images can provide them to this chart.
This was brought up as part of a support call: https://gitlab.com/gitlab-org/distribution/team-tasks/issues/512 (confidential)
And is part of helping the charts be feasible on isolated networks: #1613
Add a document and example values file that outlines how to go about changing all our image repositories and tags. Rather than having to have people hunt through various docs.