It's now already there since it's in base.
@ -1,7 +1,6 @@
FROM jitsi/base
RUN \
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 install -t stretch-backports -y prosody && \
apt-cleanup && \