This is what I can remember:
curl -L j.mp/bash-completion-docker | sh
or if you dont like curl-pipe, do it by hand:
ln -s /Applications/Docker.app/Contents/Resources/etc/docker.bash-completion /usr/local/etc/bash_completion.d/docker
ln -s /Applications/Docker.app/Contents/Resources/etc/docker.bash-completion /usr/local/etc/bash_completion.d/docker |