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>

nginx-flask-mysql/backend/Dockerfile

@@ -1,7 +1,7 @@
 FROM python:3.8-alpine
 WORKDIR /code
 COPY requirements.txt /code/
-RUN pip install -r requirements.txt
+RUN pip install -r requirements.txt --no-cache-dir
 COPY . /code/
 ENV FLASK_APP hello.py
 CMD flask run --host=0.0.0.0