KVM guest can't access internet
Solution 1:
I received a comment on Reddit that pointed me in the right direction with this one: https://www.reddit.com/r/linuxadmin/comments/bdy6sz/kvm_guest_cant_access_internet/
The problem is that Docker changes the default policy for the FORWARD chain in iptables to DROP.
A rule to allow traffic has to be added. Running this command added the required rule and fixed the issue:
sudo iptables -I FORWARD -i br0 -o br0 -j ACCEPT