This is a temporary workaround to disable auto updates. As we move out of the alpha there will be a nicer method.
There is a single simple script called "update-engine-reboot-manager" that does an automatic reboot after update-engine applies an update to your CoreOS machine. To stop automatic reboots after an update has been applied you need to stop this daemon. You can do this via a service file that is started at boot.
Create a file called /media/state/units/stop-reboot-manager.service
that has the following contents:
[Unit]
Description=stop update-engine-reboot-manager
[Service]
Type=oneshot
ExecStart=/usr/bin/systemctl stop update-engine-reboot-manager
[Install]
WantedBy=local.target
sudo systemctl enable --runtime /media/state/units/stop-reboot-manager.service
sudo systemctl start stop-reboot-manager.service
Have fun!