[docker-compose] append directory to PHP_INI_SCAN_DIR environment variable

$ cat docker-compose.yml
services:
     web:
         image: php:apache
         environment:
             - PHP_INI_SCAN_DIR=$HOME/etc/php:$ORIGINAL

$ docker-compose up -d
WARNING: The ORIGINAL variable is not set. Defaulting to a blank string.

How do I recover the original (current) $PHP_INI_SCAN_DIR?
In other words, instead of $ORIGINAL what should I place?