Earlier today I was trying to download and apply the latest updates to my Debian system (which is a Debian GNU/Linux testing stretch), and when I was doing an apt-get update I was getting this error:
W: GPG error: https://apt.dockerproject.org/repo debian-stretch InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY F76221572C52609D W: The repository 'https://apt.dockerproject.org/repo debian-stretch InRelease' is not signed. N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use. N: See apt-secure(8) manpage for repository creation and user configuration details.
I went to the oficial documentation site and all I was missing to do was a
$ curl -fsSL https://yum.dockerproject.org/gpg | sudo apt-key add -
and that was all.
Hope this helps.