Fix external renderers example

[johanvdw]

• 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).

[codecov-commenter]

Codecov Report

Merging #12841 into master will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #12841      +/-   ##
==========================================
- Coverage   43.17%   43.17%   -0.01%     
==========================================
  Files         654      654              
  Lines       72218    72218              
==========================================
- Hits        31182    31179       -3     
- Misses      35985    35987       +2     
- Partials     5051     5052       +1     

Impacted Files	Coverage Δ
modules/util/timer.go	42.85% <0.00%> (-42.86%)	⬇️
modules/log/file.go	73.60% <0.00%> (-1.61%)	⬇️
modules/queue/workerpool.go	58.77% <0.00%> (-1.23%)	⬇️
services/pull/check.go	47.69% <0.00%> (-0.77%)	⬇️
services/pull/pull.go	42.03% <0.00%> (+0.46%)	⬆️
modules/log/event.go	59.43% <0.00%> (+1.88%)	⬆️

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