The new version of Docker libnetwork executes two proxy commands: moby/moby#41908 (comment)

rootlesskit-docker-proxy -proto tcp -host-ip 0.0.0.0 -host-port 8080 -container-ip 172.17.0.2 -container-port 80
rootlesskit-docker-proxy -proto tcp -host-ip :: -host-port 8080 -container-ip 172.17.0.2 -container-port 80

rootlesskit-docker-proxy now fails with EADDRINUSE bcause it cause the equivalent of rootlessctl add-ports 0.0.0.0:8080:80/tcp [::]:8080:80/tcp

$ rootlessctl add-ports 0.0.0.0:8080:80/tcp [::]:8080:80/tcp
1
error: listen tcp 0.0.0.0:8080: bind: address already in use

rootlesskit-docker-proxy probably needs to be modified to cause rootlessctl add-ports 0.0.0.0:8080:80/tcp4 [::]:8080:80/tcp6

cc @thaJeztah @arkodg