Based on the additional context from:
This is unlikely to work as-is.
Assuming you have reconfigured docker0 to be 10.30.10.0/24, then the command @stgraber suggests will add a static route on your LXD host that will direct packets for 10.30.10.0/24 into the LXD managed bridge internal.
However in order for packets to arrive at the container with the docker0 bridge it will require that the container with the docker0 bridge responds to ARP requests for 10.30.10.0/24 on its eth0 interface.
By default Linux will respond to ARP requests for IPs bound to any of its interfaces, meaning that you should be able to reach 10.30.10.1 (the docker0 bridge address).
For just reaching the 10.30.10.1 IP this may be enough.
However any docker container with an IP in 10.30.10.0/24 connected to the docker0 bridge will not be known by the container and thus it will not respond to ARP requests on its eth0.
What you would need to reach the entire of 10.30.10.0/24 from the LXD host is to add a static route that specifies that packets be forwarded directly to the LXD container’s IP and not depend on ARP resolution of the targetted IP.
To do this you would need to run a manual command such as:
sudo ip r add 10.30.10.0/24 via <LXD container's IP> dev internal
LXD does not currently support NIC level routing for bridged NICs, the ipv{n}.routes settings behave the same as the equivalent settings on the bridge network itself.