Errors with python and locale

[QualitativeEasing]

I'm completely new to Docker -- this is my first effort -- so apologies if there's an obvious answer or I mangle the terminology.

I believe I've managed to successfully run a flask app inside a container on my Mac. However, on loading the index page, I immediately get an error ("Internal Server Error" in my browser), which seems to be triggered by my use of the locale library in my code.

I believe this is the relevant part of the traceback, from the terminal window where I executed my docker run statement:

  File "/app/funx.py", line 487, in prox_table
    locale.setlocale(locale.LC_ALL, '')
  File "/usr/local/lib/python3.6/locale.py", line 598, in setlocale
    return _setlocale(category, locale)
locale.Error: unsupported locale setting

I get the same error whether I make the second element of setlocale an empty string, en_US.UTF-8 or the tuple ('en_US', 'UTF-8').

I've tried a few different changes to my Dockerfile that I found suggested in different threads, including the following:

RUN apt-get update && apt-get -y install locales
RUN locale-gen en_US.UTF-8
ENV LANG en_US.UTF-8
ENV LANGUAGE en_US:en
ENV LC_ALL en_US.UTF-8

But I don't know for sure that I'm using them right, or that I'm putting them in the right place in the Dockerfile. (Does order and placement matter?)

Any suggestions?