Install xindy to support generating PDF with non-ascii indexes

Author: humitos

Dockerfile

@@ -106,11 +106,15 @@ RUN apt-get -y install \
 #
 # swig: is required for different purposes
 # https://github.com/rtfd/readthedocs-docker-images/issues/15
+#
+# xindy: is useful to generate non-ascii indexes
+# https://github.com/rtfd/readthedocs.org/issues/4454
 RUN apt-get -y install \
       imagemagick \
       librsvg2-bin \
       plantuml \
-      swig
+      swig \
+      xindy
 
 # Install Python tools/libs
 RUN apt-get -y install \