Transformed to Python 3, Dockerfile used python:3-alpine image, added docker-compose file

[JoelKle]

• transformed server.py to python 3
• used the official python:3-alpine image in the Dockerfile
• added a docker-compose file with description howto use it in README.md
• moved all the flask files to the ./mha dir

[JoelKle]

@lnxg33k Do you want to merge this PR?

[ninoseki]

@lnxg33k I just tested @JoelKle's PR with Python 3.8.2 and it works without any problems. It's good to use Python 3 because Python 2 has reached EOL.