fix DNS inside vpn container

modules/vpn-container.nix

···
       networking = {
         useHostResolvConf = false;
         firewall.checkReversePath = "loose";
+        nameservers = config.networking.nameservers;
+      };
+
+      services.resolved = {
+        enable = true;
+        dnssec = "true";
+        dnsovertls = "true";
+        domains = [ "~." ];
+        fallbackDns = [ ];
       };
 
       systemd.network = {