Append info on PHP_OS in the template of `admin/maintenance_env.php` and in `include/functions.inc.php` -> `send_piwigo_infos()`
Detection works by checking if PHP is running on Linux then check if PID2 is kthreadd
Kthreadd is useless in a container so PID2 should not exist of be another process
If unable to read /proc/2/sched for some reason, assume a SELinux restriction and that PHP is not running in a container
`is_in_container()` doesn't differentiate between VMs or bare metal, it only check if PHP is running in a containerized environement via tools like docker/podman/....
