Maybe I have to write the file with nix?

nixos/server/package-configs/cinny/default.nix

@@ -6,13 +6,24 @@
   ...
 }: let
   port = 2143;
+  configFile =
+    pkgs.writeText "config.json"
+    ''
+            {
+          "defaultHomeserver": 2,
+          "homeserverList": [
+              "matrix.gladtherescake.eu"
+          ],
+          "allowCustomHomeservers": false
+      }
+    '';
 in {
   #TODO: set default server and disable changing that
   virtualisation.oci-containers.containers."cinny" = {
     autoStart = true;
     ports = ["${toString port}:80"];
     volumes = [
-      "./config.json:/app/config.json"
+      "${configFile}:/app/config.json"
     ];
     image = "ajbura/cinny:latest";
   };