At the moment this is a quick and dirty built, but it works. It is based on the fullinstall-script provided by the NagiosXI team, so as long as they don 't change to much stuff, it will keep working.
You can start the container with the command:
docker run -d -p 80:80 -p 5666:5666 -p 5667:5667 --name nagiosxi tgoetheyn/docker-nagiosxi
Afterwards you can access the console at:
http://YOUR_IP/nagiosxi/
Finish the installation wizard and enjoy!
You can safelly ignore the "SSH" error and "ntpd" warning in Nagios. it's normal because the container doesn't have SSH enabled. Also ntpd can 't run, because it cannot change the system time.
For licensing, change to "free license" to keep using the product. The free license implice a 7 host limit.