Auto update to version 5.2.9.

Changelog: https://community.ubnt.com/t5/UniFi-Updates-Blog/UniFi-5-2-9-is-released/ba-p/1694199

Dockerfile

 FROM ubuntu:xenial 
 MAINTAINER Chris Smith <chris87@gmail.com> 
 
-ARG url=http://www.ubnt.com/downloads/unifi/5.2.7/unifi_sysvinit_all.deb
+ARG url=https://www.ubnt.com/downloads/unifi/5.2.9/unifi_sysvinit_all.deb
 
 RUN \
   apt-get update && \