Docker on Ubuntu - not working

Today I've installed Docker into my Ubuntu Linux 15.04 according to official documentation (note here: after installation I've executed  sudo usermod -aG docker <my_username_here> because I want to execute Docker as a non-root user).

However, when I tried to do docker docker run hello-world I got:

ernestas@ernestas-VirtualBox:~$ docker run hello-world