docker-flarum/rootfs/flarum/app/config.php

<?php return array (
  'debug' => <DEBUG>,
  'database' => array (
    'driver' => 'mysql',
    'host' => '<DB_HOST>',
    'database' => '<DB_NAME>',
    'username' => '<DB_USER>',
    'password' => '<DB_PASS>',
    'charset' => 'utf8mb4',
    'collation' => 'utf8mb4_unicode_ci',
    'prefix' => '<DB_PREF>',
    'strict' => false,
  ),
  'url' => '<FORUM_URL>',
  'paths' => array (
    'api' => 'api',
    'admin' => 'admin',
  ),
);