added step to explicitly copy requirements.txt, this optimises the build process, so just a code change does not cause a rebuild of requirements/packages

Dockerfile

@@ -5,8 +5,9 @@ ARG USERID
 ARG GROUPID
 RUN apt-get dist-upgrade -y
 WORKDIR /code
-COPY . .
+COPY requirements.txt .
 RUN pip3 install -r requirements.txt
+COPY . .
 RUN chown -R ${USERID}:${GROUPID} /code
 EXPOSE 80
 CMD gunicorn --bind=0.0.0.0:80 --workers=2 --threads=2 main:app --env ENV="production" --error-logfile /code/gunicorn.error.log --access-logfile /code/gunicorn.log --capture-output