judgeenv: properly specify a default in docker

[Riolku]

What we probably want to do here is have a default when we are in docker. Right now, if the model_file exists, the default is completely ignored.

[codecov-commenter]

Codecov Report

Merging #1027 (11ac821) into master (9551b06) will increase coverage by 2.98%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1027      +/-   ##
==========================================
+ Coverage   81.24%   84.23%   +2.98%     
==========================================
  Files         140      140              
  Lines        4789     4789              
==========================================
+ Hits         3891     4034     +143     
+ Misses        898      755     -143     

Impacted Files	Coverage Δ
dmoj/judgeenv.py	56.84% <ø> (ø)
dmoj/judge.py	53.98% <0.00%> (+1.22%)	⬆️
dmoj/executors/java_executor.py	86.36% <0.00%> (+3.03%)	⬆️
dmoj/cptbox/compiler_isolate.py	55.31% <0.00%> (+6.38%)	⬆️
dmoj/cptbox/tracer.py	76.32% <0.00%> (+18.02%)	⬆️
dmoj/cptbox/handlers.py	100.00% <0.00%> (+26.31%)	⬆️
dmoj/cptbox/isolate.py	89.63% <0.00%> (+38.41%)	⬆️
dmoj/control.py	100.00% <0.00%> (+64.70%)	⬆️

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