jvteleco
I just remembered the issue the other way round, that Docker fails to start with VPN enabled, if bridged networking is enabled: https://dietpi.com/forum/t/vpn-breaks-docker/3207/2
If you don’t require it (inter-container communication) you can disable it as mentioned in the link.
I guess it depends on the start order. When starting WireGuard first and Docker afterwards, probably Docker fails, with WireGuard first and Docker afterwards, probably WireGuard fails as you describe it. The latter is default on DietPi since WireGuard starts on earlier boot stage and Docker at very last. I hope disabling bridged network makes Docker skip iptables rules.