nginx-flask-mongo: add dev envs support (#268)

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

nginx-flask-mongo/compose.yaml

@@ -10,13 +10,20 @@ services:
       - 80:80
     depends_on:
       - backend
+
   backend:
-    build: flask
-    environment: 
+    build:
+      context: flask
+      target: builder
+    # flask requires SIGINT to stop gracefully
+    # (default stop signal from Compose is SIGTERM)
+    stop_signal: SIGINT
+    environment:
       - FLASK_SERVER_PORT=9091
     volumes:
       - ./flask:/src
     depends_on:
       -  mongo  
+
   mongo:
     image: mongo