Set locale C.UTF-8 in Dockerfile

[vjik]

Q	A
Is bugfix?	❌
New feature?	❌
Breaks BC?	❌

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.96%. Comparing base (31c2cd9) to head (f3c8af3).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #435   +/-   ##
=========================================
  Coverage     76.96%   76.96%           
  Complexity       29       29           
=========================================
  Files            11       11           
  Lines           178      178           
=========================================
  Hits            137      137           
  Misses           41       41           

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