prosody: fix installing dependdencies

Only install from backports what's absolutely necessary and make sure
libssl is installed so JWT auth is not broken...

Ref: https://github.com/jitsi/docker-jitsi-meet/issues/613

prosody/Dockerfile

@@ -8,26 +8,27 @@ 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 \
-      liblua5.2-dev \
+    && apt-dpkg-wrap apt-get install -y \
+      prosody \
+      libssl1.0.2 \
       sasl2-bin \
       libsasl2-modules-ldap \
+      lua-basexx \
+      liblua5.2-dev \
       libsasl2-dev \
-      libsssl1.0.2 \
       libssl1.0-dev \
-      lua-basexx \
-      lua-ldap \
-      lua-sec \
       luarocks \
       git \
       gcc \
       patch \
+    && apt-dpkg-wrap apt-get install -t stretch-backports -y \
+      lua-ldap \
+      lua-sec \
     && luarocks install cyrussasl 1.1.0-1 \
     && luarocks install lua-cjson 2.1.0-1 \
     && luarocks install luajwtjitsi 1.3-7 \
     && luarocks install net-url 0.9-1 \
-    && apt-dpkg-wrap apt-get remove -t stretch-backports -y \
+    && apt-dpkg-wrap apt-get remove --purge -y \
       git \
       gcc \
       luarocks \