Upgrade from repository is not possible if gitlab is stopped

If gitlab-ce is stopped, apt-get dist-upgrade fails to upgrade package:

$> apt-get dist-upgrade

The following packages will be upgraded:
  gitlab-ce

Get:1 https://packages.gitlab.com/gitlab/gitlab-ce/ubuntu xenial/main amd64 gitlab-ce amd64 8.9.6-ce.0 [280 MB]

Preparing to unpack .../gitlab-ce_8.9.6-ce.0_amd64.deb ...
gitlab preinstall: Automatically backing up only the GitLab SQL database (excluding everything else!)
rake aborted!
Errno::ENOENT: No such file or directory - connect(2) for /var/opt/gitlab/redis/redis.socket

gitlab preinstall: 
gitlab preinstall: Backup failed! If you want to skip this backup, run the following command and
gitlab preinstall: try again:
gitlab preinstall: 
gitlab preinstall:   sudo touch /etc/gitlab/skip-auto-migrations
gitlab preinstall: 
dpkg: error processing archive /var/cache/apt/archives/gitlab-ce_8.9.6-ce.0_amd64.deb (--unpack):
 subprocess new pre-installation script returned error exit status 1

Implementation Guide

We should add a note in the prerequisites list that GitLab should be running when performing an upgrade.