Improve CI performance

[hmenke]

• Trigger precompilation of Julia packages during Docker build (c1fd2be)

  We might have to revert this. I think this is causing these newly appearing build failures, although it's a bit weird that they only show up on Ubuntu.

  ┌ Warning: The call to compilecache failed to create a usable precompiled cache file for PairInteraction [top-level]
  │   exception = ArgumentError: Invalid header in cache file /github/home/.julia/compiled/v1.0/PairInteraction.ji.
  └ @ Base loading.jl:9[69](https://github.com/pairinteraction/pairinteraction/actions/runs/4367243567/jobs/7649525158#step:7:70)
  ERROR: LoadError: MethodError: no method matching setConservedParityUnderInversion(::PairInteraction.SystemTwoAllocated, ::getfield(Core.Compiler, Symbol("##252#255")){Array{Int64,1}})
  Closest candidates are:
    setConservedParityUnderInversion(::Union{SmartPointer{T2}, T2} where T2<:PairInteraction.SystemTwo, !Matched::PairInteraction.parity_t) at none:0
  Stacktrace:
   [1] top-level scope at none:0
   [2] include at ./boot.jl:317 [inlined]
   [3] include_relative(::Module, ::String) at ./loading.jl:1044
   [4] include(::Module, ::String) at ./sysimg.jl:29
   [5] exec_options(::Base.JLOptions) at ./client.jl:266
   [6] _start() at ./client.jl:425
  in expression starting at /__w/pairinteraction/pairinteraction/build/pairinteraction.jl/test/test_green_tensor.jl:45
  

• Reduce Docker image size to speed up container initialization

  Initialize containers                     1m 10s
  

• Tweak the GUI test to perform a much faster calculation

        Start 24: python.gui
  33/33 Test #24: python.gui .......................   Passed  308.08 sec
  

• Notebooks need to be sped up considerably

        Start 30: notebook.introduction
  29/37 Test #30: notebook.introduction ........................   Passed    3.47 sec
        Start 31: notebook.matrix_elements
        Start 32: notebook.vdw_near_surface
  31/37 Test #31: notebook.matrix_elements .....................   Passed    2.03 sec
        Start 33: notebook.wavefunctions
  32/37 Test #33: notebook.wavefunctions .......................   Passed    1.93 sec
        Start 34: notebook.comparison_to_saffman_fig13
  33/37 Test #32: notebook.vdw_near_surface ....................   Passed    2.54 sec
        Start 35: notebook.pair_potential_efield_sensitivity
  34/37 Test #34: notebook.comparison_to_saffman_fig13 .........   Passed   13.26 sec
        Start 36: notebook.pair_potential_near_surface
  35/37 Test #35: notebook.pair_potential_efield_sensitivity ...   Passed   49.45 sec
        Start 37: notebook.atom_ion_interaction
  36/37 Test #36: notebook.pair_potential_near_surface .........   Passed   57.88 sec
  37/37 Test #37: notebook.atom_ion_interaction ................   Passed   26.12 sec