Remotely unlock LUKS via SSH
How to unlock LUKS root partition via SSH
On Debian 9 (stretch), yet again.
apt -y install dropbear-initramfs
cp my-ssh-key /etc/dropbear-initramfs/authorized_keys
echo 'DROPBEAR_OPTIONS="-p 2222"' >> /etc/dropbear-initramfs/config
update-initramfs -u
and then connect with
ssh -p 2222 root@yourhost cryptroot-unlock
Woohoo!