dev/ci: add documentation/tips for replicating test runs using candidate images

Created by: bobheadxi

Our more troublesome tests are almost always those that run against a Sourcegraph instance. Many of these tests (notable exception: #25344 (closed) ) already use candidate images to spin up Sourcegraph instances - simply using these same images for debugging can be a significant time saver, and reduce other variables that might affect an attempted reproduction.

The only caveat at the moment is that candidate images require auth:

gcloud auth login
gcloud auth configure-docker

Perhaps we could make these public instead?