Allow option to keep the containers alive

It would be nice to keep the docker containers alive for speeding up the test runs. Now, in every test run containers are re-created and can't keep them alive since this block

  def __del__(self):
        """
        Try to remove the container in all circumstances
        """
        if self._container is not None:
            try:
                self.stop()
            except:  # noqa: E722
                pass

removes containers when the instance variable is deleted (when the program terminates) and I cannot override it because this also runs without using with as block.

I don't want my Mysql container to be recreated everytime. I am running tests very frequently and I am waiting couple seconds everytime. Pretty annoying.

I can make a small PR if that makes sense?

まず del メソッド周辺の testcontainers/core/container.py を読み、with ブロックの外での使用も含めて、テスト実行の終了時にコンテナがどのように停止されるかを追跡します。コンテキスト管理されるコンテナと直接作成されるコンテナの両方について、オプションのライフサイクル動作を定義します。完了の条件は、呼び出し側が自動クリーンアップなしで実行間に MySQL コンテナを存続させられることです。