Before be sure you properly upgraded to containers 0.3.0. Clean the old installation with sudo uninstall-container.sh -k and then install the new version. Then start containers service container system start. Check if it downloaded the latest vminit image. container i ls
If not then try  sudo uninstall-container.sh -d that deletes everything including the user data.
Setting the conainer subnet seems to be only necessary for macOS 15. Lookup the subnet of the bridge network with ifconfig. Look for the bridge, e.g.
bridge100: flags=8a63<UP,BROADCAST,SMART,RUNNING,ALLMULTI,SIMPLEX,MULTICAST> mtu 1500
        options=63<RXCSUM,TXCSUM,TSO4,TSO6>
        ether be:d0:74:63:61:64
        inet 192.168.206.1 netmask 0xffffff00 broadcast 192.168.206.255