Update image to allow shibboleth SP 3.0.x to install dependencies from stretch-backports

Dockerfile

@@ -5,7 +5,9 @@ LABEL name="LIGO GraceDB Django application" \
 ARG SETTINGS_MODULE="config.settings.container.dev"
 
 COPY docker/SWITCHaai-swdistrib.gpg /etc/apt/trusted.gpg.d
+COPY docker/backports.pref /etc/apt/preferences.d
 RUN echo 'deb http://pkg.switch.ch/switchaai/debian stretch main' > /etc/apt/sources.list.d/shibboleth.list
+RUN echo 'deb http://deb.debian.org/debian stretch-backports main' > /etc/apt/sources.list.d/backports.list
 RUN curl -sL https://deb.nodesource.com/setup_8.x | bash -
 RUN apt-get update && \
     apt-get install --install-recommends --assume-yes \

docker/backports.pref

+Explanation: shibboleth 3.0 dependencies
+Package: init-system-helpers libxerces-c3.2
+Pin: release a=stretch-backports
+Pin-Priority: 500