root can undo that with echo /sbin/modprobe > /proc/sys/kernel/modprobe at any time so you are better off using:
echo 1 > /proc/sys/kernel/modules_disabled
echo 1 > /proc/sys/kernel/modules_disabled
If you're root in a container, but not root on the outside system, you shouldn't be able to write to /proc/sys/kernel/modprobe, no?
Disabling module loading is not reversible, you need to reboot.