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://repo.aptly.info/system-tests/archive.debian.org/debian-archive/debian/dists/stretch/InRelease
Error (retrying): HTTP code 404 while fetching http://repo.aptly.info/system-tests/archive.debian.org/debian-archive/debian/dists/stretch/InRelease
Retrying 0 http://repo.aptly.info/system-tests/archive.debian.org/debian-archive/debian/dists/stretch/InRelease...
Download Error: http://repo.aptly.info/system-tests/archive.debian.org/debian-archive/debian/dists/stretch/InRelease
Downloading: http://repo.aptly.info/system-tests/archive.debian.org/debian-archive/debian/dists/stretch/Release
Downloading: http://repo.aptly.info/system-tests/archive.debian.org/debian-archive/debian/dists/stretch/Release.gpg
openpgp:  RSA key ID 04EE7237B7D453EC
openpgp: Can't check signature: public key not found
openpgp:  RSA key ID 648ACFD622F3D138
openpgp: Can't check signature: public key not found
openpgp:  RSA key ID 0E98404D386FA1D9
openpgp: Can't check signature: public key not found
openpgp:  RSA key ID EF0F382A1A7B6500
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