On a read-only filesystem creating /root/.ssh might fail, but that's ok.
Do not fail the run, as this is only needed to add the credential, which
is a separate step.
With this, I can now easily do:
systemd-nspawn --load-credential=ssh.authorized_keys.root:/home/lennart/.ssh/authorized_keys --image=… --boot
To boot into an image with my SSH key copied in. Yay!