message: nydus-image: only support single SOURCE file for create subcommand
  Only allow single SOUECE file for the 'nydus-image create' subcommand,
  there's no support of multiple SOURCE files at all.
diff: @@ -161,7 +161,7 @@ fn prepare_cmd_args(bti_string: String) -> ArgMatches<'static> {
  Arg::with_name("SOURCE")
  .help("source to build the RAFS filesystem from")
  .required(true)
  - .multiple(true),
  + .multiple(false),
  )
  .arg(
  Arg::with_name("type")
  