Detailed on the ArchWiki below is a solution to an issue I was having when I tried to use a network bridge br0 alongside Docker on my server.

The solution that I thought worked initially was to add the below to /etc/docker/daemon.json. This got the bridge working for virtual machines, but broke Docker container external connectivity (containers couldn’t access the internet).

{
  "bridge": "br0"
}

In the end I needed to run the iptables rule instead, and that got things working for both Docker and the Virtual Machines.

iptables -I FORWARD -i br0 -o br0 -j ACCEPT