Create a /bin/sh wrapper to your docker container and then point PHP Intellisense at the wrapper:
#!/bin/sh
docker run \
--rm \
-i \
--network=host \
-v "$HOME":"$HOME":ro \
-u $(id -u) \
-w "$PWD" \
vfac/envdevphpbase:7.2-cli \
php "$@"
exit $?
Please note you will need to map in any volumes you require, i've assumed your $HOME dir here. Also you must use the interactive flag -i.
{
"php.executablePath": "/somewhere/php7wrapper",
"php.suggest.basic": false
}