Set docker locale to utf-8

Shocked, shocked, that there's a Linux distribution out that there isn't
doing the right thing and setting up utf-8 by default. (Many tests failed)

Dockerfile

@@ -11,6 +11,7 @@ RUN useradd docker \
 
 # Update system and install our dependencies
 RUN apt-get update && apt-get install -y --no-install-recommends \
+  locales \
   ghostscript \
   tesseract-ocr \
   tesseract-ocr-deu tesseract-ocr-spa tesseract-ocr-eng tesseract-ocr-fra \
@@ -23,6 +24,13 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
   python3-reportlab \
   python3-pil
 
+# Enforce UTF-8
+# Borrowed from https://index.docker.io/u/crosbymichael/python/ 
+RUN dpkg-reconfigure locales && \
+  locale-gen C.UTF-8 && \
+  /usr/sbin/update-locale LANG=C.UTF-8
+ENV LC_ALL C.UTF-8
+
 # Set up a Python virtualenv and take all of the system packages, so we can
 # rely on the platform packages rather than importing GCC and compiling them
 RUN pyvenv /appenv \