Container image info are not updated properly when container is rebuilt as empty

[gabrielmougard]

Required information

• Distribution: Ubuntu
• Distribution version: 20.04

Issue description

Let say we have a stopped container c1 using whatever image. Let say we want to rebuild it as empty with lxc rebuild c1 --empty
This should work but then if we do lxc config c1 show, we see that the old image.* fields of the container aren't updated properly. This could be misleading as we can't distinguish an "empty" container (no rootfs) from a normal one.

Instead we should have no image.* fields or at least a value that stands out to let the user know that this instance won't start up because it is empty.