Hi @dz00te, great shot!
Did you also try the version from extras repository?
I installed cockpit-0.77-3.1.el7.x86_64
On your command list I'd add fw_ to the service key, to be compliant with http://docs.nethserver.org/en/latest/third_party.html#firewall
config set fw_cockpit service TCPPort 9090 access public status enabledWOW, successfully started a nethserver/docker-demo container from Cockpit 