how to correctly chain multiple files in '$settings['container_yamls'][]'?
The settings.php file already contains the following line.

$settings['container_yamls'][] = __DIR__ . '/services.yml';


It is much easier to add the following one at the bottom of the settings.php file.

$settings['container_yamls'][] = 'modules/redis/example.services.yml';


Since it is added on the bottom of the file, it is easier to find and remove it, in the case you decide not to use that module anymore. In this case, you just remove a single line, and Drupal doesn't use that module's services anymore.

Also, if you install another module that requires to add a similar line, you just need to add a $settings['container_yamls'][] which can be easily removed without touching the other lines.

Those are the only considerations I could make about which code snippet I would use. Otherwise, for PHP 5.5 (and Drupal), the following code snippets are perfectly equivalent.

$settings['container_yamls'][] = __DIR__ . '/services.yml';
$settings['container_yamls'][] = 'modules/redis/example.services.yml';




$settings['container_yamls'] = array(
  __DIR__ . '/services.yml',
  'modules/redis/example.services.yml',
);




$settings['container_yamls'] = [
  __DIR__ . '/services.yml',
  'modules/redis/example.services.yml',
];