use volumes instead of copying application files

This change drastically reduces the amount of rebuilding required during
application development and debugging stages.

Dockerfile

@@ -9,6 +9,4 @@ COPY ./requirements.txt /app/requirements.txt
 
 RUN python -m pip install -r requirements.txt
 
-COPY ./app/ /app/app/
-
 CMD flask run -h 0.0.0.0 -p $PORT

docker-compose.yaml

@@ -10,3 +10,5 @@ services:
       PORT: "12402"
     ports:
       - "12402:12402"
+    volumes:
+      - ./app:/app/app