GitLab CE docker image in Dockerhub is not built on Debian stable release

GitLab CE docker image in Dockerhub registry (owner GitLab) is based on Debian stretch/sid instead of stable Jessie release.

$ docker exec _<my GitLab container based on 8.14.5-ce.0>_ cat /etc/debian_version
stretch/sid

When attempting to build an image based on 8.15.0-ce.0, our cron installation fails, we get an error that is mentionned as fixed in Debian Jessie.

I have also noticed that the Dockerfile on Dockerhub https://hub.docker.com/r/gitlab/gitlab-ce/~/dockerfile/ is not up to date, as it builds on Ubuntu.