* libffi-dev is required for building jupyter
* matplotlib can not be installed using wheels on the used version of alpine
  linux, which means it must be compiled and  a large number of other packages
  have to be installed as well.
  This is very inefficient: see eg.
  https://pythonspeed.com/articles/alpine-docker-python/

  Apart from that, matplotlib is actually not required for rendering
  notebook files in gitea and it will pull in other dependencies which take some
  time to build (ie numpy).