[Feature request] Add Docker image for GAD app

[kat-kan]

Apart from existing deployment possibilities, could you please add a Docker image for the app?

Profits:

• course participants/GAD users could practise working with Docker

[Freudenberger]

Docker image added: https://hub.docker.com/repository/docker/jaktestowac/gad/general

Ive also updated README.md with simple instruction how to run it 😉

Cheers!