password-protect KG2c lite JSON download file; provide password via a file to Docker build

Now that we have modified PloverDB to pull the KG2c lite JSON file using HTTPS from our webserver instance (see #19), we need to eventually password-protect that webserver with basic HTTP authentication. This issue is a reminder to do that. The tricky part is to ensure that the password is provided (via some kind of secrets file) to the Jenkins build system for the ITRB-deployed PloverDB.