.env is intended for per-environment configuration and sensitive credentials.
You can maintain a config with this type of data. There is no default config for Lumen like there is for Laravel, so you need
to add the required config directory, and tell Lumen to use it:
$ mkdir config
$ touch config/app.php
In the app.php file, you can return an array with the config:
// config/app.php
<?php
return [
'order_by' => 'whatever'
// and so on
];
Then tell Lumen to load the config from app.php inside bootstrap/app.php, by adding (after $app has been initialised):
$app->configure('app');
Finally, you can use the config() helper method to get the configuration:
config('app.order_by') // whatever