Instead of `curl $url | apt-get add -` use the ubuntu keyserver and a full
fingerprint to retreive the key. This makes the distribution of the GPG key
more secure an less likely to change even if 3rd party gains access to the
packages repository.
Docker-DCO-1.0-Signed-off-by: Jonas Pfenniger <zimbatm@zimbatm.com> (github: zimbatm)