ssh-agent-passgen is a small ruby script that accesses ssh-agent to generate a reproducible random passwords based on a seed value and an SSH key. For a given seed and SSH key - the password will always stay the same (it is generated by digitally signing the seed). It also means there's no need for password storage.
The key must be unlocked within ssh-agent to be able to (re)generate the password.
docker run --rm --volume $SSH_AUTH_SOCK:/ssh-agent --env SSH_AUTH_SOCK=/ssh-agent ssh-agent-passgen $@