add nginx-flask-mysql application sample

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

samples/nginx-flask-mysql/backend/Dockerfile

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+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

samples/nginx-flask-mysql/backend/hello.py

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+from flask import Flask
+app = Flask(__name__)
+
+@app.route('/')
+def hello_world():
+    return 'Hello world'

samples/nginx-flask-mysql/backend/requirements.txt

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+click==6.7
+Flask==1.0.2
+itsdangerous==0.24
+Jinja2==2.10
+MarkupSafe==1.0
+Werkzeug==0.14.1