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

samples/nginx-flask-mysql/db/password.txt

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+db-78n9n

samples/nginx-flask-mysql/docker-compose.yaml

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+version: "3.7"
+services:
+  backend:
+    build: backend
+    depends_on: 
+    - db
+  db:
+    environment:
+      MYSQL_DATABASE: example
+      MYSQL_ROOT_PASSWORD_FILE: /run/secrets/db-password
+    image: mysql:5.7
+    restart: always
+    secrets:
+    - db-password
+    volumes:
+    - db-data:/var/lib/mysql
+  proxy:
+    build: proxy
+    ports:
+    - 80:80
+    depends_on: 
+    - backend
+volumes:
+  db-data: {}
+secrets:
+  db-password:
+    file: db/password.txt

samples/nginx-flask-mysql/proxy/Dockerfile

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+FROM nginx:1.13-alpine
+COPY conf /etc/nginx/conf.d/default.conf

samples/nginx-flask-mysql/proxy/conf

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+server {
+    listen       80;
+    server_name  localhost;
+    location / {
+        proxy_pass   http://backend:5000;
+    }
+
+}