expose DNS port via Dockerfile rather than 'docker run' argument

That is "the Docker way".

Related to #141.

weave

@@ -358,7 +358,7 @@ case "$COMMAND" in
         shift 1
         check_not_running $DNS_CONTAINER_NAME $DNS_IMAGE
         create_bridge
-        CONTAINER=$(docker run --privileged -d --name=$DNS_CONTAINER_NAME --expose=53/udp -v /var/run/docker.sock:/var/run/docker.sock $DNS_IMAGE "$@" | tail -n 1)
+        CONTAINER=$(docker run --privileged -d --name=$DNS_CONTAINER_NAME -v /var/run/docker.sock:/var/run/docker.sock $DNS_IMAGE "$@" | tail -n 1)
         with_container_netns $CONTAINER attach $CIDR >/dev/null
         echo $CONTAINER
         ;;

weavedns/Dockerfile

@@ -1,5 +1,5 @@
 FROM scratch
-# EXPOSE 53/udp
 WORKDIR /home/weave
 ADD ./weavedns /home/weave/
+EXPOSE 53/udp
 ENTRYPOINT ["/home/weave/weavedns", "-wait", "5", "-iface", "ethwe"]