extra/docker is currently a very old system tray app for openbox2, kde3 and gnome2. It hasn't been updated in a very, very long time.
docker (https://www.docker.io/) on the other hand, is a widely used and current container solution. As of 0.7, it's now available on all distributions (previously was only available on ubuntu).
There will be a naming conflict between the two very soon. I would be in favour of renaming extra/docker preemptively since it hasn't been updated in a decade.
J. Leclanche