Close #717

Add documentation changes to:

 * Set the permissions of the unix socket properly
 * Create the directory for the socket
 * Make sure tmpfiles.d persists said directory

The first change makes sure gitlabl can connect to the socket. The latter two
avoid redis from failing to start on systemd-based systems, including recent
versions of Debian.

doc/install/installation.md

@@ -150,6 +150,17 @@ We recommend using a PostgreSQL database. For MySQL check [MySQL setup guide](da
 
     # Enable Redis socket for default Debian / Ubuntu path
     echo 'unixsocket /var/run/redis/redis.sock' | sudo tee -a /etc/redis/redis.conf
+    # Grant permission to the socket to all members of the redis group
+    echo 'unixsocketperm 770' | sudo tee -a /etc/redis/redis.conf
+
+    # Create the directory which contains the socket
+    mkdir /var/run/redis
+    chown redis:redis /var/run/redis
+    chmod 755 /var/run/redis
+    # Persist the directory which contains the socket, if applicable
+    if [ -d /etc/tmpfiles.d ]; then
+      echo 'd  /var/run/redis  0755  redis  redis  10d  -' | sudo tee -a /etc/tmpfiles.d/redis.conf
+    fi
 
     # Activate the changes to redis.conf
     sudo service redis-server restart