docs: def as_string(self):
          """Return the command as a single string for the docker file"""
          if type(self.instruction) is str:
              return self.instruction
  
          if self.action == "FROM" and not isinstance(self.command, six.string_types):
              extra = "" if self.extra is NotSpecified else " {0}".format(self.extra)
              return "{0} {1}{2}".format(self.action, self.command.from_name, extra)
          else:
              return "{0} {1}".format(self.action, self.command)