How to SFTP/SSH from inside a Docker Container

  aptible, docker, sftp, ssh-agent

I have been working on a file transfer feature. Our application is deployed with a Docker image on a hosting service called Aptible. The file transfer is currently implemented with SFTP and public key authentication.
I am having a problem with Docker Containers generated based on the image because the containers do not start ssh-agent automatically on Container launches. ssh-agent is required for the authentication. I am trying to setup some scheduled job to generate and send files through SFTP so I would need ssh-agent to always run inside these Docker Containers.

I have these in my Dockerfile but it does not seem to start ssh-agent inside the containers

RUN mkdir -p /root/.ssh
RUN set -a && . /app/.aptible.env && echo "$PRIVATE_KEY" > /root/.ssh/id_rsa
RUN set -a && . /app/.aptible.env && echo "$PUBLIC_KEY" > /root/.ssh/
RUN chmod 600 /root/.ssh/id_rsa

RUN eval "$(ssh-agent -s)"

Source: Docker Questions