[docker] error_log() not work

$ cat master/.htaccess
RewriteEngine On
RewriteRule ^(.*)$ src/index-debug.php [L]

$ cat docker-compose.yml
services:
     web:
         build: .
         ports:
             - 8014:80
         volumes:
             - .:/var/www/html
             - ./docker/volumes/etc/php.ini:/usr/local/etc/php/php.ini

$ cat Dockerfile
FROM php:apache
RUN a2enmod rewrite

$ cat master/src/index-debug.php
<?php
error_log("error message 123");
var_dump(
     ini_get('error_log'),
     ini_get('log_errors'),
     file_get_contents('/var/log/error.log'),
);

$ cat docker/volumes/etc/php.ini
log_errors = On
error_log = /var/log/error.log

$ docker-compose up -d
WARNING: Found orphan containers (xxx) for this project. If you removed 
or renamed this service in your compose file, you can run this command 
with the --remove-orphans flag to clean it up.
Starting xxx... done

$ curl http://localhost:8014/master/src/index-debug.php
string(18) "/var/log/error.log"
string(1) "1"
<b>Warning</b>:  file_get_contents(/var/log/error.log): Failed to open 
stream: No such file or directory in 
<b>/var/www/html/master/src/index-debug.php</b> on line <b>4</b><br />
bool(false)

Why?

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