Added --no-cache-dir option to all Python pip commands in Dockerfiles (#113)

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>

nginx-flask-mongo/flask/Dockerfile

@@ -2,6 +2,6 @@ FROM python:3.7
 
 WORKDIR /src
 COPY . .
-RUN pip install -r requirements.txt
+RUN pip install -r requirements.txt --no-cache-dir
 
 CMD ["./server.py"]