container text-center

    <div class="container">
      <p class="text-center"><button type="button" class="btn btn-primary">モーダルボタン</button></p>
      <!-- モーダル -->
      <div class="modal" id="myModal" tabindex="-1" role="dialog" aria-labelledby="exampleModalLabel"
        aria-hidden="true">
        <!-- モーダルのダイアログ本体 -->
        <div class="modal-dialog" role="document">
          <!-- モーダルのコンテンツ -->
          <div class="modal-content">
            <div class="modal-header">
              <h5 class="modal-title" id="exampleModalLabel">モーダルのタイトル</h5>
              <button type="button" class="close" data-dismiss="modal" aria-label="Close"> <span
                  aria-hidden="true">×</span> </button>
            </div>
            <div class="modal-body">モーダルの本文</div>
            <div class="modal-footer">
              <button type="button" class="btn btn-secondary" data-dismiss="modal">閉じる</button>
              <button type="button" class="btn btn-primary">変更を保存</button>
            </div>
          </div>
        </div>
      </div>
    </div>
    <script>
      $('.btn').click(function () {
        $('#myModal').modal('show');
      });
    </script>

    <div class="container">
      <p class="text-center my-5"><button type="button" class="btn btn-primary" data-toggle="modal"
          data-target="#myModal">モーダルボタン</button></p>
      <div class="modal" id="myModal" tabindex="-1" role="dialog" aria-labelledby="exampleModalLabel"
        aria-hidden="true">
        <div class="modal-dialog" role="document">
          <div class="modal-content">
            <div class="modal-header">
              <h5 class="modal-title" id="exampleModalLabel">モーダルのタイトル</h5>
              <button type="button" class="close" data-dismiss="modal" aria-label="Close"> <span
                  aria-hidden="true">×</span> </button>
            </div>
            <div class="modal-body">モーダルの本文が入ります。</div>
            <div class="modal-footer">
              <button type="button" class="btn btn-secondary" data-dismiss="modal">閉じる</button>
              <button type="button" class="btn btn-primary">変更を保存</button>
            </div>
          </div>
        </div>
      </div>
    </div>
    <script>
      $('#myModal').on('hide.bs.modal', function (e) {
        if (!confirm('閉じてよろしいですか？')) {
          e.preventDefault();
        }
      });
    </script>
</body>
</html>