Docker containers don't have a locale set by default - the approach proposed in PEP 528 actually comes from the way I configure Docker images (which in turn comes from Armin Ronacher's recommendations in click for Python 3 locale handling).

In the Dockerfile for Fedora based containers I add:

    ENV LC_ALL=C.UTF-8
    ENV LANG=C.UTF-8

while in CentOS 7 based containers I add:

    ENV LC_ALL=en_US.UTF-8
    ENV LANG=en_US.UTF-8

And with those settings, Python 3 based containers just work (my laptop is running en_AU.UTF-8 locally)