The build we are doing here goes only into the temporary registry, and
is then pulled back locally. So this image only needs the tag that it
will get pulled with, which is done just below -- we can remove the
"tag_prefix" and buildset-registry tags that won't be used.
Change-Id: Ia9a3d3476e7971ee5a094f74628a6f6575b535a9
f4ffc94694