When using docker buildx to build a container image, use a temporary
registry to receive the built image instead of requiring a buildset
registry.
A multi-arch test is also added with a publication registry
using the same task list to reduce duplication.
Change-Id: Ib20d1c97f6cb63e0ff9d8888ea792d1941cd8690
Co-Authored-By: James E. Blair <jeblair@redhat.com>