Add support for download of world in zipfile

WORLD=http://hostname/world.zip
Zipfile should contain world/ directory, and optionally any required mods in mods/ as well as optionally
a default server.properties file (overwritten by any env vars)
Also export volumes /mods and /config, the contents of which are copied to /data/mods and /data/config.  This
allows shared read-only mounts for modules without affecting world.

minecraft-server/Dockerfile

@@ -12,7 +12,9 @@ RUN wget -O /usr/bin/jsawk https://github.com/micha/jsawk/raw/master/jsawk
 RUN chmod +x /usr/bin/jsawk
 RUN useradd -M -s /bin/false --uid 1000 minecraft \
   && mkdir /data \
-  && chown minecraft:minecraft /data
+  && mkdir /config \
+  && mkdir /mods \
+  && chown minecraft:minecraft /data /config /mods
 
 EXPOSE 25565
 
@@ -20,6 +22,8 @@ COPY start.sh /start
 COPY start-minecraft.sh /start-minecraft
 
 VOLUME ["/data"]
+VOLUME ["/mods"]
+VOLUME ["/config"]
 COPY server.properties /tmp/server.properties
 WORKDIR /data
 
@@ -32,4 +36,4 @@ ENV UID=1000
 ENV MOTD A Minecraft Server Powered by Docker
 ENV JVM_OPTS -Xmx1024M -Xms1024M
 ENV TYPE=VANILLA VERSION=LATEST FORGEVERSION=RECOMMENDED LEVEL=world PVP=true DIFFICULTY=easy \
-  LEVEL_TYPE=DEFAULT GENERATOR_SETTINGS=
+  LEVEL_TYPE=DEFAULT GENERATOR_SETTINGS= WORLD=