Add Attic binary cache and containerize gitea runner

Replace nix-serve-only setup with Attic for managed binary caching with
upstream filtering and GC. Move gitea actions runner from host into an
isolated NixOS container with private networking. nix-serve kept alongside
Attic during migration.

common/server/default.nix

@@ -12,6 +12,7 @@
     ./mailserver.nix
     ./nextcloud.nix
     ./gitea-actions-runner.nix
+    ./atticd.nix
     ./librechat.nix
     ./actualbudget.nix
     ./unifi.nix