I just encountered the same thing, and after a bit of head-scratching got things working. I suspect your problem is this:
Originally Posted by zinnium
The 5 IPs are set to the main OS and should forward to the device as well.
I also configured the spare IP I wanted to use in the host OS as a virtual eth0:X interface.. as soon as I removed it from /etc/network/interfaces and rebooted (you may not need to reboot, but I seem to recall downing an eth0:X interface took eth0 with it last time I tried)... then the container picked up that IP no problem, and networking from inside started to work.
I assumed, the host OS would do all the routing but did need to have that IP assigned to it.. but seems like that was the thing getting in the way.
Hope this helps with your problem too.