Refactor docker setup

[jaumebecks]

Refactored docker setup:

• Moved docker config to a docker/ folder
• Added ability to build images per environment with the same Dockerfile
• Added BUILD_ENV as .envvar
• Created DEV environment

Performed tests:

• docker-compose -f docker/docker-compose.yml build tests works (using requirements-test.txt)
• make build works OR docker-compose -f docker/docker-compose.yml build python works (using requirements-dev.txt if BUILD_ENV is set in .env, and requirements-prod.txt the other way around)
• envvars still exist in containers
• docker-compose volumes still works (updated code outside a running container and was properly updated in the container)

[jaumebecks]

Fixed conflicts and polished Makefile

[jaumebecks]

Is there anything else to be done @joanfont ?