using multiple pubkeys to update authorized_keys file

[sidorasrl]

Beside generating a new RSA key (with associated public key) to get ssh connections among containers I find useful to register other public keys.
My first approach was to manage an authorized_keys file in the src/ssh/ folder but now I think a better solution is to manage additional public keys while updating the authorized_keys list.
A minor change in src/ssh/sshd_start is all is needed.