code: func DockerClientVersion(ssh SSHCommander) (string, error) {
  	// `docker version --format {{.Client.Version}}` would be preferable, but
  	// that fails if the server isn't running yet.
  	//
  	// output is expected to be something like
  	//
  	//     Docker version 1.12.1, build 7a86f89
  	output, err := ssh.SSHCommand("docker --version")
  	if err != nil {
  		return "", err
  	}
  
  	words := strings.Fields(output)
  	if len(words) < 3 || words[0] != "Docker" || words[1] != "version" {
  		return "", fmt.Errorf("DockerClientVersion: cannot parse version string from %q", output)
  	}
  
  	return strings.TrimRight(words[2], ","), nil
  }
docs: // DockerClientVersion returns the version of the Docker client on the host
  // that ssh is connected to, e.g. "1.12.1".