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`.

The issue names the `rie` entrypoint and a docker-compose `command` using `app.handler --watch`, but no repository files or tests. Start by locating the command entrypoint and server launch path, then determine how file changes in a volume can trigger a restart. Done means the flag works in the shown container setup and reruns the server after source changes.

索引モデルが issue の本文から書いたものです。