[docker] error_log() not write inside web page

supaplex$ cat docker-compose.yml
services:
     web:
         image: php:apache
         ports:
             - 8013:80
         volumes:
             - .:/var/www/html

supaplex$ cat index.php
<?php
echo "BEFORE\n";
error_log("abc");
echo "AFTER\n";

supaplex$ docker-compose up -d
supaplex_web_1 is up-to-date

$ docker-compose run web php index.php
Creating supaplex_web_run ... done
BEFORE
abc
AFTER

OK, but...

supaplex$ curl http://localhost:8013/
BEFORE
AFTER

Why *abc* does not appear?