The pertinent parts of lxc.conf for coral access and igpu encoder.
features: keyctl=1,nesting=1
unprivileged: 1
dev0: /dev/dri/renderD128,gid=106
lxc.cgroup2.devices.allow: c 189:* rwm
lxc.mount.entry: /dev/bus/usb dev/bus/usb none bind,optional,create=dir
106 is the render group in my debian...