base: add backports repository

base/Dockerfile

@@ -16,6 +16,7 @@ RUN \
 	apt-key add /tmp/jitsi.key && \
 	rm -f /tmp/jitsi.key && \
 	echo "deb https://download.jitsi.org $JITSI_RELEASE/" > /etc/apt/sources.list.d/jitsi.list && \
+	echo "deb http://ftp.debian.org/debian stretch-backports main" > /etc/apt/sources.list.d/backports.list && \
 	apt-dpkg-wrap apt-get update && \
 	apt-dpkg-wrap apt-get dist-upgrade -y && \
 	apt-dpkg-wrap apt-get autoremove -y --purge gnupg && \