add matplotlib in validation/doc docker

2 jobs for master in 35 minutes and 53 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
failed #13998
makedg

00:35:53

failed #13999
pages

00:05:34

 
Name Stage Failure
failed
makedg Test There is an unknown failure, please try again

98% tests passed, 1 tests failed out of 41

Total Test time (real) = 1721.14 sec

The following tests FAILED:
9 - modules/atmosphere/tests/GravityWavesSphereParallel (Failed)
Errors while running CTest
ERROR: Build failed: exit code 1
failed
pages Test There is an unknown failure, please try again
$ pydoc3 -w slim slimPre slimPost slim3d
wrote slim.html
wrote slimPre.html
problem in slimPost - ImportError: No module named 'matplotlib'
wrote slim3d.html
$ mkdir -p ../public
$ mv slim.html slimPre.html slimPost.html slim3d.html ../public
mv: cannot stat 'slimPost.html': No such file or directory
ERROR: Build failed: exit code 1