Don't suggest exposing DOCKER_AUTH_CONFIG to student repos

[kalabukdima]

This token, if not masked, can be seen by students and allows sensitive data leaks. It is only used in runners setup, not in the CI jobs, hence it should never be set in the group configuration.

[codecov]

Codecov Report

Merging #31 (44eadbb) into main (6389346) will increase coverage by 8.31%.
Report is 12 commits behind head on main.
The diff coverage is 64.07%.

@@            Coverage Diff             @@
##             main      #31      +/-   ##
==========================================
+ Coverage   36.94%   45.25%   +8.31%     
==========================================
  Files          24       25       +1     
  Lines        1903     2022     +119     
==========================================
+ Hits          703      915     +212     
+ Misses       1200     1107      -93     

Flag	Coverage Δ
unittests	45.25% <64.07%> (+8.31%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
checker/__main__.py	0.00% <0.00%> (ø)
checker/actions/check.py	0.00% <0.00%> (ø)
checker/actions/contributing.py	0.00% <ø> (ø)
checker/actions/export.py	0.00% <0.00%> (ø)
checker/actions/grade.py	0.00% <0.00%> (ø)
checker/actions/grade_mr.py	0.00% <0.00%> (ø)
checker/executors/sandbox.py	87.77% <ø> (ø)
checker/utils/git.py	17.50% <ø> (ø)
checker/utils/manytask.py	80.85% <ø> (+62.48%)	⬆️
checker/testers/make.py	44.44% <20.00%> (+44.44%)	⬆️
... and 13 more