marukuru: tweak GitLab Puma settings for low-mem

Per the GitLab docs
(https://docs.gitlab.com/ee/install/requirements.html#puma-settings):

> If the operating system has a maximum 2 GB of memory, the recommended number
> of threads is 1. A higher value will result in excess swapping, and decrease
> performance.

and

> In a memory-constrained environment with less than 4GB of RAM available,
> consider disabling Puma Clustered mode.
>
> Configuring Puma by setting the amount of workers to 0 could reduce memory
> usage by hundreds of MB. For details on Puma worker and thread settings, see
> Puma settings.

ops/nixos/marukuru/default.nix

@@ -148,6 +148,10 @@ in {
           gitlab_rails['gitlab_email_reply_to'] = 'noreply@hg.lukegb.com'
 
           prometheus['enable'] = false
+
+          puma['min_threads'] = 1
+          puma['max_threads'] = 1
+          puma['worker_processes'] = 0
         '';
       };
     };