4027: Revamp SSH use in our Docker images.

4027: Revamp SSH use in our Docker images.

* Don't install or run SSH in most of our Docker images.  `docker
  exec` is now preferred to inspect running images.

* Do run SSH on the API server, always, for Gitolite.

There is a feature regression here: the user's SSH key is not
automatically installed on the shell account.  This needs to be fixed
another way.  In the meantime, it's not difficult to run
`docker exec -ti --user=self shell /bin/bash`, and you can clone the
repository from the host system.