Description:
Unintended/unexpected privilege escalation to root privilege
from the docker package.

Additional information:
Docker version:
Client version: 1.2.0
Client API version: 1.14
Go version (client): go1.3.1
Git commit (client): fa7b24f
OS/Arch (client): linux/amd64
Server version: 1.2.0
Server API version: 1.14
Go version (server): go1.3.1
Git commit (server): fa7b24f

OS: Linux 3.16.3-1-ARCH #1 SMP PREEMPT Wed Sep 17 21:54:13 CEST 2014
x86_64 GNU/Linux

Steps to reproduce:
1) Folow the installation process https://wiki.archlinux.org/index.php/Docker#Installation
2) Folow the configuration process https://wiki.archlinux.org/index.php/Docker#Configuration
3) Logout and logon the user member of docker
4) docker pull image debian:latest
5) issue: docker run -t -i -v /:/host-root /bin/bash
6) You are root on your host FS without any sudo, just because you are member of docker
7) Modify /etc/passwd and /etc/shadow to set a root account

Patch or warning:
- Do not allow docker to be issued without sudo
- Warn the users in the Wiki page that members of "docker" must be more than trusted and
can get root access on the host machine