Docker has experimental support for building multi-arch
container images with a buildx command. Currently it only
supports pushing to a registry after running and the images
don't end up in the local docker images list. To work around
that, push to the buildset registry then pull back. This
is the inverse of the normal case where we build, then
retag, then push. The end result should be the same.
Change-Id: I6a4c4f9e262add909d2d5c2efa33ec69b9d9364a
63bd307e63