ref: https://docs.docker.com/installation/ubuntulinux/#ubuntu-trusty-1404-lts-64-bit
Ubuntu-maintained Package Installation
To install the latest Ubuntu package (this is not the most recent Docker release):
$ sudo apt-get update
$ sudo apt-get install docker.io
Then, to enable tab-completion of Docker commands in BASH, either restart BASH or:
$ source /etc/bash_completion.d/docker.io
==> 예전 버전의 경우 docker exec 가 안되므로, 아래 최신 버전 사용 권장
Docker-maintained Package Installation
Add the Docker repository key to your local keychain.
$ sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 36A1D7869245C8950F966E92D8576A8BA88D21E9
Add the Docker repository to your apt sources list, update and install the lxc-docker package.
You may receive a warning that the package isn't trusted. Answer yes to continue installation.
$ sudo sh -c "echo deb https://get.docker.com/ubuntu docker main > /etc/apt/sources.list.d/docker.list"
$ sudo apt-get update
$ sudo apt-get install lxc-docker