Update Dockerfile.response

This reduces the final response image size from ~1gb to ~230mb by using the `slim` python image and removing unnecessary apt caching.

Author: marcosnils

demo/Dockerfile.response

@@ -1,7 +1,8 @@
-FROM python:3.7
+FROM python:3.7-slim
 
-RUN apt-get update && apt-get install -y \
-    netcat
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    netcat \
+    && rm -rf /var/lib/apt/lists/*
 
 WORKDIR /app
 COPY requirements.txt /app
@@ -10,4 +11,4 @@ RUN pip install -r requirements.txt
 COPY . /app/
 
 ENTRYPOINT ["python", "manage.py"]
-CMD ["runserver", "0.0.0.0:8000"]
+CMD ["runserver", "0.0.0.0:8000"]