Path to file containing the secret key base. The content of the file will be sourced into SECRET_KEY_BASE environment variable. The secret has a minimum length requirement of 64 bytes.
One way to generate such a secret is to use openssl rand -hex 64.
This file is loaded using systemd credentials, and therefore does not need to be owned by the greenlight user.
If this option is null, it will be created at /var/lib/greenlight/secrets/secret-key-base with a new secret key base.