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