Fix port to listen on, and add simple Dockerfile

dns/Dockerfile

@@ -0,0 +1,3 @@
+FROM ubuntu
+EXPOSE 53/udp
+ADD ./dns /home/dns

dns/main.go

@@ -80,5 +80,5 @@ func main() {
 
 	go dns.ActivateAndServe(nil, conn, MDNSServeMux)
 
-	dns.ListenAndServe(":5300", "udp", LocalServeMux)
+	dns.ListenAndServe(":53", "udp", LocalServeMux)
 }