As far as I can tell, your app logged a few lines, the error seems to be elsewhere?
2021-10-31 01:39:01,449 INFO supervisord started with pid 510
2021-10-31 01:39:02,451 INFO spawned: 'nginx' with pid 516
2021-10-31 01:39:02,453 INFO spawnerr: can't find command 'php-fpm8'
2021-10-31 01:39:02,590 INFO gave up: php-fpm entered FATAL state, too many start retries too quickly
2021-10-31 01:39:03,592 INFO success: nginx entered RUNNING state, process has stayed up for > than 1 seconds (startsecs)
It wouldn’t have been able to log these lines if it couldn’t write to /dev/stdout or /dev/stderr.