opengpg: failure opening keyring '${HOME}/.gnupg/aptlytest.gpg': open ${HOME}/.gnupg/aptlytest.gpg: no such file or directory

Looks like your keyring with trusted keys is empty. You might consider importing some keys.
Downloading http://cdn-fastly.deb.debian.org/debian/dists/stretch/InRelease...
Downloading http://cdn-fastly.deb.debian.org/debian/dists/stretch/Release...
Downloading http://cdn-fastly.deb.debian.org/debian/dists/stretch/Release.gpg...
openpgp:  RSA key ID 7638D0442B90D010
openpgp: Can't check signature: public key not found
openpgp:  RSA key ID 04EE7237B7D453EC
openpgp: Can't check signature: public key not found
ERROR: unable to fetch mirror: failed to verify detached signature: openpgp: signature made by unknown entity