Get no such file error after installing Docker

I am getting the following error when I try to run a docker command like 'docker images':

[ec2-user@ip-172-31-21-104 ~]$ docker images
FATA[0000] Get http:///var/run/docker.sock/v1.18/images/json: dial unix /var/run/docker.sock: no such file or directory. Are you trying to connect to a TLS-enabled daemon without TLS? 

This is a fresh AWS instance, updated, with docker installed out-of-the-box (no mods). I start the docker service. No errors. I run a docker command and get this error.

What am I missing?