gitlab: Increase shared buffer size to 8GB

Gitlab recommends a default value of 1/4th of the total memory here, but
since the previous value was 1MB we go for a bit more conservative
approach here.

Fixes: #615


Signed-off-by: Christian Heusel <christian@heusel.eu>

roles/gitlab/tasks/main.yml

@@ -97,6 +97,7 @@
           }
         ]
         prometheus_monitoring['enable'] = false
+        postgresql['shared_buffers'] = "8GB"
     volumes:
       - "/srv/gitlab/config:/etc/gitlab"
       - "/srv/gitlab/logs:/var/log/gitlab"