We have a chicken/egg problem with ignition and installing a root ssh key on a
transactional-update system:

/root is not mounted in initrd, you need to add an entry in the ignition config
to mount /root. To do that, you need to know the device name. Which leads to
two problems:
1. I don't know the device name upfront, I need to login first. But's that's
not possible without working ignition config.
2. The device name varies on different machines. So you need an extra
config/usb disk for every different machine.