See https://docs.docker.com/engine/userguide/eng-image/dockerfile_best-practices/#apt-get
- Replaced `-y install` with `install -y` (unification)
- Put every package on its own line
- Moved `rm -rf /var/lib/apt/lists/*` in the same `RUN` statement as `apt-get`
- Removed unnecessary `apt-get clean`
See 03e2923e42/contrib/mkimage/debootstrap (L82-L105)