zulip/docs/development/setup/install-docker.md

2. Add yourself to the docker group:

$ sudo adduser $USER docker
Adding user `YOURUSERNAME' to group `docker' ...
Adding user YOURUSERNAME to group docker
Done.

You will need to reboot for this change to take effect. If it worked, you will see docker in your list of groups:

$ groups | grep docker
YOURUSERNAME adm cdrom sudo dip plugdev lpadmin sambashare docker

3. Make sure the Docker daemon is running:

If you had previously installed and removed an older version of Docker, an Ubuntu bug may prevent Docker from being automatically enabled and started after installation. You can check using the following:

$ systemctl status docker
● docker.service - Docker Application Container Engine
   Loaded: loaded (/lib/systemd/system/docker.service; enabled; vendor preset: enabled)
   Active: active (running) since Mon 2019-07-15 23:20:46 IST; 18min ago

If the service is not running, you'll see Active: inactive (dead) on the second line, and will need to enable and start the Docker service using the following:

$ sudo systemctl unmask docker
$ sudo systemctl enable docker
$ sudo systemctl start docker