Added --no-cache-dir option to all Python pip commands in Dockerfiles

The cache gives no benefit to a Docker image but it does add size, hence it's best left out.

Signed-off-by: Andy Madge <andymadge@gmail.com>

flask/app/Dockerfile

@@ -1,7 +1,7 @@
 FROM python:3.7-alpine 
 WORKDIR /app 
 COPY requirements.txt /app
-RUN pip3 install -r requirements.txt 
+RUN pip3 install -r requirements.txt --no-cache-dir
 COPY . /app 
 ENTRYPOINT ["python3"] 
 CMD ["app.py"]