Installation needs to define /var/run/redis in tmpfiles.d
The installation instructions recommend using redis with a unix domain socket at /var/run/redis/redis.sock.
On Debian, as well as most modern distributions, /var/run is a symlink to /run, and /run is mounted tmpfs, therefore, the directory /var/run/redis is volatile and will dissapear after rebooting, making gitlab fail, since it can't connect to redis.
The documentation needs updating, and tmpfiles.d needs to be used to create these directories.