As of 2020-08, it uses `redis:alpine` but this should instead be pinned to the oldest version we support, which as of %13.0 is `4`. (@tnir EDITED) As of 2021-04, the CI uses `redis:5` (https://gitlab.com/gitlab-org/gitlab/-/issues/238216#note_495055423) but this should instead be pinned to the oldest version we support, which as of %13.0 is `4`.