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
}
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
}
// DockerClientVersion returns the version of the Docker client on the host
// that ssh is connected to, e.g. "1.12.1".