Symlink docker.io binary to docker.

[griff]

The binary installed by the docker.io package is called docker.io so this change symlinks that to docker.

[misheska]

I would suggest doing this in a way that doesn't conflict with the existing docker systray app. I suggest this approach from Guy Baconniere via http://blog.docker.io/2014/04/docker-in-ubuntu-ubuntu-in-docker/, fleshing it out into a final, tested solution:

Guy Baconniere April 19, 2014 Use docker instead of docker.io on Ubuntu 14.04 LTS http://pastebin.com/raw.php?i=hm3y4vJy

# Docker on Ubuntu 14.04 LTS
sudo apt-get install docker.io

#
# if you prefer the command docker over docker.io 
# on Ubuntu 14.04 LTS
#

# Create /usr/bin/docker the Debian/Ubuntu way
# (avoid conflicting with docker – System tray)
sudo update-alternatives –install /usr/bin/docker docker /usr/bin/docker.io 50

# Allow bash completion for docker
sudo cp -a /etc/bash_completion.d/docker{.io,}
sudo sed -i 's/\(docker\)\.io/\1/g' /etc/bash_completion.d/docker

# Allow zsh completion for docker
sudo cp -a /usr/share/zsh/vendor-completions/_docker{.io,}
sudo sed -i 's/\(docker\)\.io/\1/g' /usr/share/zsh/vendor-completions/_docker

# the man page for docker
sudo ln -s /usr/share/man/man1/docker{.io,}.1.gz

# not really needed because docker.io is still there
sudo sed -i 's/\(docker\)\.io/\1/g' /usr/share/docker.io/contrib/*.sh

# Test Docker
sudo docker pull ubuntu
sudo docker run -i -t ubuntu /bin/bash
exit

# TODO?
# sudo dpkg -L docker.io | xargs grep -s ‘docker.io’
# /usr/share/lintian/overrides/docker.io
# /usr/share/doc/docker.io