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!

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