After upgrading Docker to version 20.10 on CentOS 8 and Fedora 33 we can see that it fails to start docker.service. In docker.service logs and firewalld.service logs we can see following message:
ERROR: ZONE_CONFLICT: 'docker0' already bound to a zone
Solution for this issue:
sudo firewall-cmd --zone=trusted --remove-interface=docker0
sudo firewall-cmd --zone=trusted --remove-interface=docker0 --permanent
sudo firewall-cmd --zone=trusted --remove-interface=docker0 --permanent