code: func shell(b *Builder, args []string, attributes map[string]bool, flagArgs []string, original string) error {
  	shellSlice := handleJSONArgs(args, attributes)
  	switch {
  	case len(shellSlice) == 0:
  		// SHELL []
  		return errAtLeastOneArgument("SHELL")
  	case attributes["json"]:
  		// SHELL ["powershell", "-command"]
  		b.RunConfig.Shell = strslice.StrSlice(shellSlice)
  		// b.RunConfig.Shell = strslice.StrSlice(shellSlice)
  	default:
  		// SHELL powershell -command - not JSON
  		return errNotJSON("SHELL")
  	}
  	return nil
  }
docs: // SHELL powershell -command
  //
  // Set the non-default shell to use.