nixos-module/container/dns: allow underscores in dynamic hostnames

nix/nixos-module/container/dns.nix

@@ -324,6 +324,11 @@ in
             algorithm hmac-sha256;
             secret "${inputs.zentralwerk-network-key.lib.dyndnsKey}";
           };
+
+          # allow underscores in dynamic hostnames
+          ${lib.concatMapStringsSep "\n" (type: ''
+            check-names ${type} ignore;
+          '') [ "master" "slave" "response" ]}
         '';
       });