docker: Install required ssh/config so migration can push to gitlab

This:
- explicits the necessary ssh-keygen generation
- explicits the docker/ssh volume mountpoint (as a result of ^)
- explicits the changes required in the config.lisp regarding reading the public key for
the migration to take it into account
- deactivates the known host check
- deactivates the writing to the ~/.ssh/known_hosts file to avoid out of sync
issues (when the gitlab state is reset once in a while).

Related to T4064