I had the same issue but was able to see an additional error in /var/log/syslog
docker: Error response from daemon: Conflict. The container name “/openhab.service” is already in use by container “76d75cd2089d587361ac5e2ddc52ed1382454bb48421477e261140e64d041a0c”. You have to remove (or rename) that container to be able to reuse that name.
Adding this line to /etc/systems/system/openhab.service worked like a charm for me:
ExecStartPre=-/usr/bin/docker rm %n
After adding that you need to reload systemd:
sudo systemctl daemon-reload
And finally ask systemd to start OpenHAB:
sudo systemctl start openhab
Just a note of caution that you need to have followed the instructions to setup your openhab docker with external storage as this wipes out your container each time it re/starts.