gitlab: build all container images during CI

gitlab: build all container images during CI

We have a number of container images in tests/docker/dockerfiles
that are intended to provide well defined environments for doing
test builds. We want our CI system to use these containers too.

This introduces builds of all of them as the first stage in the
CI, so that the built containers are available for later build
jobs. The containers are setup to use the GitLab container
registry as the cache, so we only pay the penalty of the full
build when the dockerfiles change. The main qemu-project/qemu
repo is used as a second cache, so that users forking QEMU will
see a fast turnaround time on their CI jobs.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20200622153318.751107-3-berrange@redhat.com>
[AJB: tweak the tag format]
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Acked-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20200701135652.1366-22-alex.bennee@linaro.org>