feat: update docker compose api version 3

[DahlitzFlorian]

• update the project to the latest docker-compose version
• update web service to Python 3.7
• refactored Dockerfile of nginx service

Note: The nginx Dockerfile now uses COPY instead of ADD as it's only meant to copy a certain file and not unpack or download it.

[dbader]

Awesome, thanks a ton @DahlitzFlorian!