IsContainerized returns false when executed in a container using cgroupns as /proc/1/cgroup is just 0::/ due to cgroup namespacing in the container.

Please consider also checking for /sys/fs/cgroup/cgroup.type in these cases. See for example the container detection in systemd, https://github.com/systemd/systemd/blob/main/src/basic/virt.c#L593.