Build using the containers openpgp backend

golang-github-containers-image has two implementations of gpg/pgp. One
using libgpgme and one using golangs crypto openpgp implementation.
gpgme is not in target (and has some GPL-3 related licensing concerns)
so not really useful for Apertis. Golangs openpgp implementation is
deprecated but still gets security fixes, which means it's not ideal but
a better option for Apertis

Signed-off-by: Sjoerd Simons <sjoerd@collabora.com>