tests/docker: use explicit docker.io registry

tests/docker: use explicit docker.io registry

It is good practice to use an explicit registry for referencing the base
image. This is because some distros will inject their own registries
into the search path. For example registry.fedoraproject.org comes ahead
of docker.io. Using an explicit registry avoids wasting time querying
multiple registries for images that they won't have.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20210623142245.307776-5-berrange@redhat.com>
Message-Id: <20210709143005.1554-13-alex.bennee@linaro.org>