feat(deertopia): Binary cache

- Provides a container whose traffic is routed through Mullvad VPN.
- An option `sydnix.deertopia.mullvad.container.modules` is provided to
  "hook into" the container's NixOS config.

hosts/deertopia/configuration.nix

@@ -47,6 +47,7 @@
       webdav.enable = true;
       copyparty.enable = true;
       syncthing.enable = true;
+      cache.enable = true;
 
       # A simple default webpage.  This should probably live somewhere else.
       nginx.vhosts."www" = {