🔧 Disable threading in flask debug server

For educational purposes, the RNG service is meant to
process only one request at a time (without concurrency).
But the flask server now defaults to a multi-threaded
implementation, which defeats our original purpose.
So here we disable threading to restore the original
behavior.

dockercoins/rng/Dockerfile

@@ -3,5 +3,5 @@ WORKDIR /app
 RUN pip install Flask
 COPY rng.py .
 ENV FLASK_APP=rng FLASK_RUN_HOST=:: FLASK_RUN_PORT=80
-CMD ["flask", "run"]
+CMD ["flask", "run", "--without-threads"]
 EXPOSE 80