Consider tagging QA image with the commit SHA instead of the branch name
A race condition was discovered in #335717 (comment 630112309), where a master
QA image could be more recent that the GitLab image against which it is run.
This is because we currently tag QA images with the branch name, so it's possible that a master
QA image is updated before end-to-end tests are run against a given GitLab image.