Make it possible to just build images.

There are a couple of very interesting build-only use cases. One is
building in a permissive environment in order to deploy in a hostile
one. Another is to build images as a separate CI job to minimise the
overall work done by the CI system.

Change-Id: Ibd729fa80aa68f002a55073e57ba3b7f770700b4
Merge "Make it possible to just build images."