title: Docker "unsupported locale setting" when running Python container
body: <p>When I try "docker run -p 8050:8050 app1" in docker I get:</p>
  
  <pre><code>Traceback (most recent call last):
  File "app1.py", line 6, in &lt;module&gt;
  locale.setlocale(locale.LC_ALL, 'en_US.UTF-8')
  File "/usr/local/lib/python3.6/locale.py", line 598, in setlocale
  return _setlocale(category, locale)
  </code></pre>
  
  <p>My dockerfile looks like this:</p>
  
  <pre><code>FROM python:3.6
  USER root
  WORKDIR /app
  ADD . /app
  RUN apt-get update &amp;&amp; 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
  RUN pip install --trusted-host pypi.python.org -r requirements.txt
  EXPOSE 8050
  ENV NAME World
  CMD ["python", "app1.py"]
  </code></pre>
  
  <p>How can i set a local language in the app1.py without getting this error?
  thanks in advance</p>
  
  <p>PS: Already restarted docker.</p>