Please note:

• If referring to a user's UID this way, the UID needs to be explicitly defined (see example below)
• This example also shows how to convert the UID to a string for services that need it

{ config, pkgs, lib, ... }:

{
  users.users = {
    exampleuser = {
      uid = 1005;
    };
  };

  virtualisation.oci-containers.containers = {
    exampleservice = {
      environment = { 
        PUID = builtins.toString config.users.users.exampleuser.uid;
      };
    };
  };
}