Use Composer $COMPOSER_VENDOR_DIR in autoload filepath - symfony

Question

I am using Symfony for a Dockerized app. I need to move my composer vendors directory, so I did set the environnement variable $COMPOSER_VENDOR_DIR to /root/app-vendors.
The vendors are correctly located to this directory, but it causes troubles with the Symfony VarDump component. By default in Symfony 2, it is loaded from composer like this:
"autoload-dev": {
"files": [
"vendor/symfony/symfony/src/Symfony/Component/VarDumper/Resources/functions/dump.php"
]
}
In my case, this path becomes wrong. I'd like to do something like this:
"autoload-dev": {
"files": [
"{$COMPOSER_VENDOR_DIR}/symfony/symfony/src/Symfony/Component/VarDumper/Resources/functions/dump.php"
]
},
But I don't see anything in the composer documentation about using env variables in a composer.json filepath.
I want to move the vendors in dev mode, but I want them to be in the default location if $COMPOSER_VENDOR_DIR is not set ; so I cannot put absolute filepath in autoload-dev section like this:
"autoload-dev": {
"files": [
"/root/app-vendors/symfony/symfony/src/Symfony/Component/VarDumper/Resources/functions/dump.php"
]
},

Answer 1

A simple solution after a few days of step back:
Create this file on app/dump.php
<?php
$vendorsDir = getenv('COMPOSER_VENDOR_DIR')
?: __DIR__.'/../vendor';
require "$vendorsDir/symfony/symfony/src/Symfony/Component/VarDumper/Resources/functions/dump.php";
Then autoload this file instead of the Symfony one
"autoload-dev": {
"files": [
"app/dump.php"
]
},
Note:
COMPOSER_VENDOR_DIR default is "vendor", see https://getcomposer.org/doc/03-cli.md#composer-vendor-dir