Mateusz Dukat

Add Ubuntu dependency guide. Main difference between Debian and Ubuntu is lack of `libsdl2-mixer-2.0-0` package out-of-the-box, and updated `libcsfml-audio2.4` package (version 2.3 does not exist in repositories of Ubuntu Bionic...

As of commit https://github.com/CERT-Polska/n6/commit/35117913da19c16d578a3a399ca3d00f95049287, `docker-compose.yml` contains `additional_contexts` which [requires Compose v2](https://docs.docker.com/reference/compose-file/build/#additional_contexts). This PR changes commented examples with old `docker-compose` commands, which may let users use [unsupported Compose v1](https://docs.docker.com/compose/releases/migrate/#what-are-the-differences-between-compose-v1-and-compose-v2), with newer...