I heard of people trying to run docker on lxc. Didn't work out too well for them. :)
Yes, and it appears that the easiest way to do that is just to run an ordinary Ubuntu VM on your ProxMox cluster and then run Docker Engine on that. Basically just bring up a clean OS build and then follow the instructions at https://www.citadel.org/docker.html to deploy Citadel. It ought to be ridiculously easy.