Change gitlab socket file location to $gdk_root

gdk thin was not working as expected anymore in my setup. The service started up correctly (according to console output and logs) but I always ended up with connection refused in the browser - it seems, it wasn't able to find gitlab.socket. Turns out the socket file location was Dir.pwd.

I needed to make the change in this MR to make it work again (in my setup).

There might of course be other issues with this approach I am not seeing right now. Please let me know if there is something I am overlooking here 🙇

(Might be related to #681 (closed))