Currently the builder script uses a specific appliance image tag from quay.io. This creates confusion especially during the developments, since the appliance image tags are not versioned and there's not any way to correlate them to a specific OCP release branch.

As we're approaching the 4.20/4.21 branch cut date (and thus soon entering in the new development branch), it will be necessary for the builder to use the right appliance image version, based on the release branch.

This is a preliminary step not only to ensure that (potentially unstable/conflicting) patches on the appliance main will be used for releasing an ISO based on a lower OCP version, but also to properly support the standard OCP bugfix backporting process