I had issues setting up the bridge, before I realized that bridge-netfilter filtering must be turned off.

Symptoms

• Can ping external server from host
• Can ping DHCP server (in Docker) from host
• Cannot ping host from external server or vice-versa

Solution
Disable bridge netfiltering on the host.

# sysctl -w net.bridge.bridge-nf-call-iptables=0
# sysctl -w net.bridge.bridge-nf-call-ip6tables=0

Could you add this as a hint to the README?