cat /path/to/public_key | ssh root@yourdokkuinstance "sudo sshcommand acl-add dokku [description]"
[description]can be anything as long as it is one word only (e.g. "personal", "home", etc.).
I have a problem, I started using dukke I was able to add ssh key, but when wanting to add another key using git and git push dokku master, I asked for a password I had lost 3 days in the forum everything is confusing ..