docs: def pair(self):
          """return (container_port, (ip, host_port)) or (container_port, host_port)"""
          if self.ip is NotSpecified:
              if self.ip is NotSpecified:
                  second = self.host_port
              else:
                  second = (self.ip, )
          else:
              second = (self.ip, self.host_port)
          return self.container_port.port_str, second