I know this questions has been answered, however the reason this is happening to you, was probably because you did not add your username to the docker group.
Here are the steps to do it:
Add the docker group if it doesn't already exist:
sudo groupadd docker
Add the connected user ${USER}
to the docker group. Change the user name to match your preferred user:
sudo gpasswd -a ${USER} docker
Restart the Docker daemon:
sudo service docker restart
If you are on Ubuntu 14.04-15.10* use docker.io instead:
sudo service docker.io restart
(If you are on Ubuntu 16.04 the service is named "docker" simply)
Either do a newgrp
docker or log out/in to activate the changes to groups.