I don't get this: it seems normal to me that the interface would be down until it's configured by the container, pretty much like on a normal machine. The only situation in which you can expect an interface to be up already is in a network-booting situation, in which the initramfs configures the interface. In a virtualised situation, this is like the host configuring the container's interface for it.
Anyway, no big deal either way for me.
Paul
You are right. Then, following your previous comments:
The host has configuration that creates a bridge br0, containing only the physical interface enp7s0. The bridge should be given the IP address that you want the host to have.
When the container is started, using --network-bridge=br0, the host automatically creates the vb-dahlia interface and adds it to the br0 bridge. No additional configuration is necessary on the host.
The container should configure its network exactly as for a normal, non-virtualised system. It can use DHCP if necessary, in which case it will receive an IP on the same network as the host. Conceptually, they are connected to the same network via a hub/switch.
I think it would be best-practice to set the network configuration inside the container, if possible.
I decided, when writing the wiki, to setup the container network static IP (example given) INSIDE the container. This approach solves the interfaces being DOWN issue for any network profile and sounds in fact a more best-practice. TY for your comments.