Fix use of locales for python et al

Dockerfile

@@ -1,6 +1,16 @@
 FROM rethinkdb:2.3
 
-RUN apt-get update
+# From http://stackoverflow.com/a/38553499
+
+RUN apt-get update && DEBIAN_FRONTEND=noninteractive apt-get install -y locales
+
+RUN sed -i -e 's/# en_US.UTF-8 UTF-8/en_US.UTF-8 UTF-8/' /etc/locale.gen && \
+    echo 'LANG="en_US.UTF-8"'>/etc/default/locale && \
+    dpkg-reconfigure --frontend=noninteractive locales && \
+    update-locale LANG=en_US.UTF-8
+
+ENV LANG en_US.UTF-8
+
 RUN apt-get -y install python3 python3-pip libffi-dev
 RUN pip3 install --upgrade pip
 RUN pip3 install --upgrade setuptools