Update to have mysql container, and usage of .env template

[techsmyth]

Description

As a user / contributor to Cherrytwist I want to have the simple demo to have the full set of containers so that I can run it

Acceptance criteria

• [x] MySql container added and networked into the docker-composer setup
• [ ] Update the readme to reflect updated setup, including block diagram visualisation of interaction between the containers.
• [x] Add template .env file for managing environment variable interactions.

Additional Context