add nginx-flask-mysql application sample

Signed-off-by: Anca Iordache <anca.iordache@docker.com>

samples/nginx-flask-mysql/backend/Dockerfile

@@ -0,0 +1,10 @@
+FROM python:3.6-alpine3.7
+EXPOSE 5000
+ENV PYTHONUNBUFFERED 1
+RUN mkdir /code
+WORKDIR /code
+ADD requirements.txt /code/
+RUN pip install -r requirements.txt
+ADD . /code/
+ENV FLASK_APP hello.py
+CMD flask run --host=0.0.0.0