Auto update to version 5.6.29.

Changelog: https://community.ubnt.com/t5/UniFi-Updates-Blog/UniFi-5-6-29-Stable-has-been-released/ba-p/2191996

Dockerfile

 FROM ubuntu:xenial 
 MAINTAINER Chris Smith <chris87@gmail.com> 
 
-ARG url=https://www.ubnt.com/downloads/unifi/5.3.8/unifi_sysvinit_all.deb
+ARG url=https://www.ubnt.com/downloads/unifi/5.6.29/unifi_sysvinit_all.deb
 
 RUN \
   apt-get update && \