Include SSH client for SSH protocol in DOCKER_HOST

[a-abella]

Docker 18.09 added SSH as a supported protocol for the Docker client DOCKER_HOST configuration. This configuration requires an SSH client to be installed. I noticed that the docker-git images include SSH, but the base images do not. As DOCKER_HOST over SSH is now built-in Docker functionality, it may be correct to include SSH with the base image in order to preserve that functionality.

[tianon]

Good catch; I tested and it adds ~6MB, which is totally reasonable, thus #207 👍