It is a good idea to add a user to docker group. In case user is not added to docker group, you may see the following error msg (even when docker daemon is running):
$ docker ps Cannot connect to the Docker daemon. Is the docker daemon running on this host?
To fix this error, ether run all docker commands as root or add current user to docker group as shown below:
$ sudo usermod -aG docker user1
After this logout and login again for changes to take effect.