Assuming you've setup ~/containers/ssh to start a sshd on boot:
sudo systemd-nspawn -D ~/containers/ssh -n -b -p 23:22
The magic is the
-p switch that allows port forwarding from the host to the container.
So when I
ssh -p 23 $host I get to my ssh container, since it's redirected to port 22. Sadly ssh does not send the hostname as part of the SSH protocol as discussed in http://serverfault.com/questions/34552/is-there-a-name-based-virtual-host-ssh-reverse-proxy.