The command mysql -h$DB_HOST -u$DB_USERNAME -p$DB_PASSWORD $DB_DATABASE works.
storage/logs/laravel-2018-09-08.log
[2018-09-08 19:10:58] production.ERROR: exception 'PDOException' with message 'SQLSTATE[HY000] [1045] Access denied for user 'cachet'@'172.17.0.7' (using password: NO)' in /var/wwwroot/default/vendor/laravel/framework/src/Illuminate/Database/Connectors/Connector.php:55 Stack trace: #0 /var/wwwroot/default/vendor/laravel/framework/src/Illuminate/Database/Connectors/Connector.php(55): PDO->__construct('mysql:host=mysq...', 'cachet', NULL, Array)