[WIP] PoC - add Docker for Monofony

[gabiudrescu]

Fixes #193

What I did:

• I took the docker configuration from SyliusStandard and adapted it to Monofony
• improved it to match some of my requirements, such as: -- not running containers as root -- solved some caching issues with docker and docker-compose by using a scratch container to have a copy of all files from context -- added redis to be able to use same session from multiple containers -- switched the hardcoded locale to an env parameter

Still to do:

• ~~add documentation~~ done
• ~~add ability to run testing suite from containers~~ decided not to tackle this now
• ~~add gitlab-ci.yml to allow ci on gitlab~~ done
• ~~move docker registry to dockerhub~~ done

[gabiudrescu]

I did this PR because I wanted to start working on https://github.com/Monofony/SymfonyStarter/pull/238 and I realized that I don't have a working environment on my laptop. And since I created a Docker setup for me, I thought it would be nice to submit it here as well.