Is missing cockpit-docker no longer in the 20.04 repository?
Though they do not build cockpit-docker for Ubuntu 20.04 or later anymore, you can still install older versions according to this.
I downloaded cockpit-docker_215-1~ubuntu19.10.1_all.deb and installed manually. It seems to be working now.
If you really want to make a difference; I forked the thing and pulled it apart as a plugin.
You can find it on Github