cron not in omnibus docker image

According to https://docs.gitlab.com/ce/raketasks/backup_restore.html#configuring-cron-to-make-daily-backups I should be able to add a backup job with "crontab -e". But the package cron isn't installed in the docker image:

$ docker run -it gitlab/gitlab-ee:11.11.0-rc1.ee.0 bash
root@635936a2e7ef:/# dpkg -l | grep cron
root@635936a2e7ef:/# which crontab
root@635936a2e7ef:/#