fix: keycloak nginx buffer size

Tune according to
https://www.getpagespeed.com/server-setup/nginx/tuning-proxy_buffer_size-in-nginx

hosts/nachtigall/apps/keycloak.nix

@@ -25,6 +25,7 @@
       "/" = {
         extraConfig = ''
           proxy_pass http://127.0.0.1:8080;
+          proxy_buffer_size 8k;
         '';
       };
     };