Mirror-registry: default database secret key stored in plain-text on initial configuration file

A flaw was found when using mirror-registry to install Quay. It uses a default database secret key, which is stored in plain-text format in one of the configuration template files. This issue may lead to all instances of Quay deployed using mirror-registry to have the same database secret key. This flaw allows a malicious actor to access sensitive information from Quay's database.