Images should include gnupg for APT

The newer Debian Buster images lack the gnupg package, preventing one from configuring private APT sources on machines lacking connectivity to the wider internet. (Seen on AWS)

# apt-key list
E: gnupg, gnupg2 and gnupg1 do not seem to be installed, but one of them is required for this operation

This should be included in the published images like it was until Stretch.