It seems you didn’t really read the documentation.
You also need to have Docker-CE installed. There are well-documented procedures for installing Docker on Ubuntu at Docker.com, you can find installation steps for your Linux distribution in the menu on the left.
Some distributions, like Ubuntu, have a
docker.iopackage available. Using that packages will cause issues! Be sure to install the official Docker-CE from the above listed URL.
Docker is not always ready with a release when a new Ubuntu version is out. Check if your version of Ubuntu is supported by docker here.