To log in without having to type the password from a Linux host:
$ ssh-keygen -t rsa
Use the default file locations and leave the password empty (provided you are the only root on your computer).
$ ssh-copy-id defaultuser@IP
Type the depeloper password one last time. After that public-key authentication should work.
Now you can become root on your Jolla and copy ~defaultuser/.ssh/authorized_keys to ~/.ssh. After that
$ ssh root@IP
should also work without asking for a password.