* 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).
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