Laravel Logging - Monolog stderr ignores logLevel

I'm using laravel 5.7 in a docker container. My goal is to write error-log directly to stderr instead of laravel.log

logging.php

<?php
return [

'default' => env('LOG_CHANNEL', 'stack'),
'channels' => [
        'stack' => [
            'driver' => 'stack',
            'channels' => ['stderr'], //before it was stderr
        ],

        'single' => [
            'driver' => 'single',
            'path' => storage_path('logs/laravel.log'),
            'level' => 'error',
        ],
//...
        'stderr' => [
            'driver' => 'monolog',
            'handler' => StreamHandler::class,
            'level' => 'error',
            'with' => [
                'stream' => 'php://stderr',
            ],
        ],
//...
    ],
];

The log target itself is working - laravel.log is empty - but it seems like the log level is ignored. In the docker logs I get also the debug messages

docker-compose logs --tail=100 container
...
ESC[36mcontainer_1         |ESC[0m [2019-07-01 09:57:51] local.DEBUG: ...
...

If I just change channels (stack) from stderr to single the logs (but only error level) are appended correctly to laravel.log

via Chebli Mohamed