Hey there.
Sorry, I did a bad job reading your full post and just focused on the stacktrace 
I’ll blame it on the holidays. 
I went ahead and tried scanning a file name with Cyrillic symbols – файл.txt… and it didn’t give me any trouble. I still couldn’t reproduce it even when following a colleague’s advice who debugged a similar issue.
Still… I would expect this is an issue of having the encoding set correctly. Where is it that are you adjusting the locale (and then running the locale command from)? Within the docker container, or only on the machine hosting the docker engine?