flask-redis: dev envs support & misc improvements (#265)

(Most of this is almost identical to #263.)

* Docker Desktop Development Environments config
* Use cache volumes for pip
* Downgrade from Python 3.11 _alpha_ -> Python 3.10
* Use port `8000` to avoid conflicts with Airplay on
    macOS for default Flask port `5000`
* Use `SIGINT` to gracefully stop Flask
* Formatting fixes in `compose.yaml`

Signed-off-by: Milas Bowman <milas.bowman@docker.com>

flask-redis/README.md

@@ -24,7 +24,7 @@ services:
    web:
         build: .
         ports:
-            - "5000:5000"
+            - "8000:8000"
         volumes:
             - .:/code
         depends_on:
@@ -55,12 +55,12 @@ Listing containers must show one container running and the port mapping as below
 $ docker compose ps
 NAME                  COMMAND                  SERVICE             STATUS              PORTS
 flask-redis-redis-1   "redis-server --load…"   redis               running             0.0.0.0:6379->6379/tcp
-flask-redis-web-1     "/bin/sh -c 'python …"   web                 running             0.0.0.0:5000->5000/tcp
+flask-redis-web-1     "/bin/sh -c 'python …"   web                 running             0.0.0.0:8000->8000/tcp
 ```
 
-After the application starts, navigate to `http://localhost:5000` in your web browser or run:
+After the application starts, navigate to `http://localhost:8000` in your web browser or run:
 ```
-$ curl localhost:5000
+$ curl localhost:8000
 This webpage has been viewed 2 time(s)
 ```
 
@@ -80,3 +80,14 @@ Stop and remove the containers
 ```
 $ docker compose down
 ```
+
+## Use with Docker Development Environments
+
+You can use this sample with the Dev Environments feature of Docker Desktop.
+
+![Screenshot of creating a Dev Environment in Docker Desktop](../dev-envs.png)
+
+To develop directly on the services inside containers, use the HTTPS Git url of the sample:
+```
+https://github.com/docker/awesome-compose/tree/master/flask-redis
+```