The installation procedure is extracted from Docker official homepage, just for my own reference.

1. Download and install the GPG Key

$ apt-get update
$ apt-get install apt-transport-https ca-certificates
$ sudo apt-key adv --keyserver hkp://p80.pool.sks-keyservers.net:80 --recv-keys 58118E89F3A912897C070ADBF76221572C52609D

2. Add the following line to /etc/apt/sources.list.d/docker.list

deb https://apt.dockerproject.org/repo ubuntu-trusty main

3. Install from Apt

$ sudo apt-get update
$ sudo apt-get install linux-image-extra-$(uname -r)
$ sudo apt-get install linux-image-generic-lts-trusty
$ sudo apt-get install docker-engine
$ sudo service docker start
$ sudo docker run hello-world

4. A docker user group is created, we add a general user to this group, the user can control docker.

$ sudo usermod -aG docker ubuntu
$ shutdown -r now