RootFS: GPG Key Error

[dack-fe]

From Pull Request #19

There are a couple of gpg key errors for some debian apt sources:

W: GPG error: http://cdn-fastly.deb.debian.org/debian jessie Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7638D0442B90D010 NO_PUBKEY CBF8D6FD518E17E1
W: GPG error: http://cdn-fastly.deb.debian.org/debian stretch Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7638D0442B90D010 NO_PUBKEY 04EE7237B7D453EC NO_PUBKEY EF0F382A1A7B6500