Merge branch 'pin-gunicorn' into 'master'

Pin Gunicorn version to avoid a bug in 20.0.0

See merge request !542

Dockerfile

@@ -26,7 +26,7 @@ RUN \
 # Install arkindex and its deps
 # Uses a source archive instead of full local copy to speedup docker build
 COPY dist/arkindex-*.tar.gz /tmp/arkindex.tar.gz
-RUN pip install /tmp/arkindex.tar.gz gunicorn && rm /tmp/arkindex.tar.gz
+RUN pip install /tmp/arkindex.tar.gz gunicorn==19.9 && rm /tmp/arkindex.tar.gz
 
 # Allow access to logs
 RUN mkdir -p /logs