forgejo-runner: set explicit DNS to fix container resolution

Runner containers (via Podman compat) couldn't resolve external hosts
after AdGuard stopped binding to 0.0.0.0. Point them at 10.0.0.1.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

services/forgejo-runner.nix

@@ -30,6 +30,7 @@
         # runner falls back to the Forgejo instance's mirror (data.forgejo.org)
         # which doesn't host most GitHub-marketplace actions.
         settings.runner.default_actions_url = "https://github.com";
+        settings.container.dns_server = "10.0.0.1";
       };
     };
   };