When one of my docker machine created virtual machines got changed by my cloud provider I was left in the dark. Out of a sudden docker-machine ssh no longer worked which comes at no surprise as the initially assigned IP address to me got changed.
In such a situation the SSH command line client has a way to specify the certificate to use when connecting to the VM:
ssh -i /home/john/.docker/machine/machines/T2fiware-postgres/id_rsa firstname.lastname@example.org
Replace `T2fiware-postgres` with the name of your docker created machine, ubuntu with the user the certificate was issued to and obviously the IP address with the one of your VM.