Add a --watch option for docker development

We are using `public.ecr.aws/lambda/python:3.8`which uses `rie` as an entrypoint.
We use docker as a local development environment, so we do not need to setup a python **virtualenv** and avoid **"it works on my machine"** errors. We map the source code into the container using a volume and at the moment the only way to get the container to the run the latest changes, is `docker-compose restart CONTAINER-NAME`.

This is not ideal and we would appreciate having a `--watch` flag added to `rie` such that we could add it to the docker-compose `command` option, e.g. `["app.handler", "--watch"]`. This flag would rerun the server inside the container upon changes, similar to Flask's `runserver`.

該 issue 提到了 `rie` 進入點和使用 `app.handler --watch` 的 docker-compose `command`，但沒有提供任何儲存庫檔案或測試。首先定位命令進入點和伺服器啟動路徑，接著判斷 volume 中的檔案變更如何觸發重新啟動。當該 flag 在所示的容器設定中生效，並在原始碼變更後重新執行伺服器時，即視為完成。

由索引模型根據 Issue 內容生成。