Multi Environment config/config.php - Staging not working

I've just updated a site from EE2.9.3 to 5.1.1 and am attempting to deploy to a staging location so my client can preview it.

I've added a switch statement to users/config/config.php in order to keep things working locally and on staging. Locally all works as expected. However on staging I get an error about database connections:

SQLSTATE[HY000] [1045] Access denied for user 'root'@'localhost' (using password: YES)
ee/legacy/database/drivers/mysqli/mysqli_connection.php:81

Here is my switch statement with real paths and user/password replaced

    switch (strtolower($_SERVER['HTTP_HOST'])) {
​
        // Live
        case 'livedomain.com' :

  // Live stuff here
​
​
        break;
​
​
​
        // staging
        case 'staging.com' :

  // Staging stuff here
     // local stuff here
  $config['cp_url'] = 'http://staging.com/admin.php';
  $config['database'] = array (
   'expressionengine' => 
   array (
    'hostname' => 'localhost',
    'password' => 'password123',
    //'user' => 'user123',
    'database' => 'dennis',
   ),
  );
  $config['site_url'] = 'http://staging.com/';
  $config['base_url'] = 'http://staging.com/';
  $config['base_path'] = 'path/to/public';
  $config['theme_folder_path'] = 'path/to/public/themes/';
  $config['theme_folder_url'] = 'http://staging.com/themes/';
  $config['new_version_check'] = 'n';

        break;
​
        // main local site
        case 'dennis.test' :
​
   // local stuff here
  $config['cp_url'] = 'http://dennis.test/admin.php';
  $config['database'] = array (
   'expressionengine' => 
   array (
    'hostname' => 'localhost',
    'password' => 'root',
    'database' => 'dennis',
   ),
  );
  $config['site_url'] = 'http://dennis.test/';
  $config['base_url'] = 'http://dennis.test/';
  $config['base_path'] = 'D:\sites\dennis-drenner';
  $config['theme_folder_path'] = 'D:\sites\dennis-drenner/themes/';
  $config['theme_folder_url'] = 'http://dennis.test/themes/';
  $config['new_version_check'] = 'y';
​
        break;
​
    }

I tried adding this 'user' => 'user123', on staging but that didn't help. I"m not sure where to set the database user - it's not in local but connects correctly and I need to override that so it works on staging.

Please advise.