just run ```build.sh Dockerfile.debian``` instead

publicize repo signing key as part of startup, previously
was only doing this at the end of the update_mirror.sh script

swiching to xenial should be as easy as modifying the dockerfile
and updating the update_mirror.sh script

If the .gnupg directory doesn't exist and we try to import keys
gpg won't be able to import the keys on that run, running --list-keys
first sets things up before we try to import