[Unit]
Description=Store OpenSSH host keys into /cfg
After=cfg.mount sshd.service
Requires=cfg.mount sshd.service
ConditionPathExists=!/cfg/etc/ssh/ssh_host_rsa_key

[Service]
Type=oneshot
RemainAfterExit=yes
ExecStartPre=/bin/mkdir -p /cfg/etc/ssh
ExecStart=/bin/sh -c 'cp -a /etc/ssh/ssh_host_*_key* /cfg/etc/ssh/'

[Install]
WantedBy=multi-user.target