Add GNUPG to Debian 9
Debian 9 appear to be the only builder image that didn't have GnuPG (gpg) installed.
Correct that.
This does not explicitly add it to other Distribution/Versions as they all have it already installed as a dependency from another package.
Related to https://gitlab.com/gitlab-org/omnibus-gitlab/issues/2537
Edited by Jason Plum