[syslog] syslog() and error_log() not work

General Discussions
1 
+ cat /etc/docker/daemon.json
{
    "log-driver": "syslog"
}

+ cat docker-compose.yml
services:
    my:
        image: php:apache
        volumes:
            - ./index.php:/var/www/html/index.php

+ cat index.php
<?php

function dump($intro, $deepening) {
    echo "$intro: $deepening\n";
}

function abc_number() {
    global $argv;

    static $number = 0;
    return "{$argv[1]}" . ++$number;
}

ini_set('error_log', 'syslog');
dump("error_log", ini_get('error_log'));

syslog(LOG_NOTICE, abc_number());
error_log(abc_number());
trigger_error(abc_number());
throw new Exception(abc_number());

+ sudo systemctl restart docker

+ docker-compose run my php index.php DOCKER-ABC
[cut]
error_log: syslog
[cut]

+ journalctl -b | grep DOCKER-ABC
[cut]: Notice: DOCKER-ABC3 in /var/www/html/index.php on line 19
[cut]: Fatal error: Uncaught Exception: DOCKER-ABC4 in /var/www/html/index.php:20

It seems that syslog() and error_log() not work.
Ineed DOCKER-ABC1 and DOCKER-ABC2 do not appear.
Why?