flask: dev envs support & misc improvements (#263)

* Docker Desktop Development Environments config
* Use cache volumes for pip
* Upgrade from Python 3.7 -> Python 3.10
* Use port `8000` to avoid conflicts with Airplay on
  macOS for default Flask port `5000`
* Use `SIGINT` to gracefully stop Flask

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

flask/.docker/docker-compose.yaml

@@ -0,0 +1,10 @@
+services:
+  web: 
+    build:
+      context: app
+      target: dev-envs
+    stop_signal: SIGINT
+    ports: 
+      - '8000:8000'
+    volumes:
+      - /var/run/docker.sock:/var/run/docker.sock