func getDockerEnv() (*dockerEnv, error) {
	dockerHost := os.Getenv("DOCKER_HOST")
	var err error
	if dockerHost == "" {
		dockerHost, err = getDefaultDockerHost()
		if err != nil {
			return nil, err
		}
	}
	dockerTLSVerify := os.Getenv("DOCKER_TLS_VERIFY") != ""
	var dockerCertPath string
	if dockerTLSVerify {
		dockerCertPath = os.Getenv("DOCKER_CERT_PATH")
		if dockerCertPath == "" {
			home := homedir.Get()
			if home == "" {
				return nil, errors.New("environment variable HOME must be set if DOCKER_CERT_PATH is not set")
			}
			dockerCertPath = filepath.Join(home, ".docker")
			dockerCertPath, err = filepath.Abs(dockerCertPath)
			if err != nil {
				return nil, err
			}
		}
	}
	return &dockerEnv{
		dockerHost:      dockerHost,
		dockerTLSVerify: dockerTLSVerify,
		dockerCertPath:  dockerCertPath,
	}, nil
}

// PrintDefaults prints, to standard error unless configured
// otherwise, the default values of all defined flags in the set.
func (fs *FlagSet) PrintDefaults() {
	writer := tabwriter.NewWriter(fs.Out(), 20, 1, 3, ' ', 0)
	home := homedir.Get()

	// Don't substitute when HOME is /
	if runtime.GOOS != "windows" && home == "/" {
		home = ""
	}

	// Add a blank line between cmd description and list of options
	if fs.FlagCount() > 0 {
		fmt.Fprintln(writer, "")
	}

	fs.VisitAll(func(flag *Flag) {
		format := "  -%s=%s"
		names := []string{}
		for _, name := range flag.Names {
			if name[0] != '#' {
				names = append(names, name)
			}
		}
		if len(names) > 0 && len(flag.Usage) > 0 {
			val := flag.DefValue

			if home != "" && strings.HasPrefix(val, home) {
				val = homedir.GetShortcutString() + val[len(home):]
			}

			fmt.Fprintf(writer, format, strings.Join(names, ", -"), val)
			for i, line := range strings.Split(flag.Usage, "\n") {
				if i != 0 {
					line = "  " + line
				}
				fmt.Fprintln(writer, "\t", line)
			}
		}
	})
	writer.Flush()
}