Wrong contents of gitlab.rb for mattermost_nginx option custom_nginx_config
In EE 8.12 gitlab.rb reads:
mattermost_nginx['custom_nginx_config'] = "include /etc/nginx/conf.d/example.conf;"
However, this is not included.
Correct syntax is only available with GitLab itself:
nginx['custom_nginx_config' ] = "...
There is only nginx.conf file, so I think this option should be removed from the sample for mattermost_nginx configuration.