I have not tried that, because I upgraded, but an ugly hack that would work is to follow my first article by using the old Keycloak image to create this admin user. Then just delete that service, keep the database and connect your Keycloak Quarkus edition to that. But maybe it is without KC_ first, could seem so according to this article: https://www.keycloak.org/server/containers
So try KEYCLOAK_ADMIN and KEYCLOAK_ADMIN_PASSWORD.