prosody: fix installing prosody from the right repository

prosody/Dockerfile

@@ -8,8 +8,8 @@ RUN \
     && rm -f /tmp/prosody.key \
     && echo "deb http://packages.prosody.im/debian stretch main" > /etc/apt/sources.list.d/prosody.list \
     && apt-dpkg-wrap apt-get update \
+    && apt-dpkg-wrap apt-get install -y prosody \
     && apt-dpkg-wrap apt-get install -t stretch-backports -y \
-      prosody \
       liblua5.2-dev \
       sasl2-bin \
       libsasl2-modules-ldap \