This has been asked several times so I'll list the steps to undo autologin.
If you don't set a root password first, you will not be able to login to the container again, ever.
Debian / Ubuntu:
1. set the root password
sudo passwd root
2. Remove Autologin
remove --autologin root from /etc/systemd/system/container-getty@1.service.d/override.conf
3. Reboot Machine
reboot
Alpine:
1. set the root password
passwd root
2. Remove Autologin
rm -rf /etc/local.d/autologin.start
3. Reboot Machine
reboot