How to update docker on Ubuntu

docker --version 

sudo su

echo deb https://apt.dockerproject.org/repo ubuntu-`cat /etc/lsb-release | grep DISTRIB_CODENAME | cut -d'=' -f2` main >> /etc/apt/sources.list.d/docker.list

apt-key adv --keyserver keyserver.ubuntu.com --recv-keys F76221572C52609D

apt-get update

apt-get purge lxc-docker*

apt-get purge docker.io*

apt-get install docker-engine

exit

docker --version