Users need to first update their PATH and set the DOCKER_HOST environment variable in their ~/.bashrc (or equivalent for other shells). These are set by default for new users, but if your workstation has been upgraded from an earlier version of Ubuntu you will need to edit your ~/.bashrc to include the following (noting that $UID in the second line will then automatically evaluate to your user ID)

PATH="$PATH:/usr/share/docker.io/contrib"
export DOCKER_HOST="unix:///run/user/$UID/docker.sock"

Note that after editing your ~/.bashrc changes will only take effect for a new login (or if you manually source ~/.bashrc )

Then run setup-user-docker (you only need to do this once). This will launch docker as a user process, and configure it to autostart when you log in.