Custom OIDC external authentication ignores PREFERRED_URL_SCHEME configuration

After correctly configuring a custom OIDC provider for my organization and authenticating against it from a running quay installation built off the qui-gon branch, the Flask app redirects my client to an http:// URL despite setting PREFERRED_URL_SCHEME: 'https' in config.yaml.

It is caused by some non-obvious features of the Flask url_for() function requiring the _scheme='https" and _external=True params

I have fixed this in a custom build and PR from my fork.