slim down docker file

[FFroehlich]

pip install should use --no-cache-dir to minimise image size.

[dweindl]

To save a couple MB more: add apt-get clean and rm -rf /var/lib/apt/lists/*

[FFroehlich]

rm -rf /var/lib/apt/lists/*

limits downstream reusability though and only saves ~200B, see https://gist.github.com/marvell/7c812736565928e602c4

[dweindl]

only saves ~200B

48M /var/lib/apt/lists/

limits downstream reusability

not quite true. just needs an apt update before installing additional apt packages.