Working through: https://github.com/knorrie/network-examples/blob/master/birdhouse-intro/README.md
Following Cloning and configuring new containers;
The container/config uses:
lxc.rootfs.path = overlay:/var/lib/lxc/birdbase/rootfs:/var/lib/lxc/weaver/delta0
However, this does not mount /var/lib/lxc/weaver/rootfs
This causes later copied data to be omitted when the container is running.
After searching and testing, I found that this configuration works correctly:
lxc.rootfs.path = overlay:/var/lib/lxc/weaver/rootfs:/var/lib/lxc/birdbase/rootfs:/var/lib/lxc/weaver/delta0
Also note, the mounting-order is Crucial, otherwise data in weaver/rootfs is still unavailable.