implement gunicorn for prod WSGI server

Dockerfile

@@ -3,4 +3,4 @@ COPY ./app /app
 WORKDIR /app
 RUN pip3 install -r requirements.txt
 ENV FLASK_APP=app.py
-CMD ["python3", "-m", "flask", "run", "--host=0.0.0.0", "--port=80"]
+CMD ["gunicorn", "-b", "0.0.0.0:80", "-w", "4", "app:app"]

app/requirements.txt

@@ -1,6 +1,9 @@
 # This file is used by pip to install required python packages
 # Usage: pip install -r requirements.txt
 
+# Gunicorn WSGI Server
+gunicorn==20.1.0
+
 # Flask Framework
 click==8.1.3
 Flask==2.2.2