All targets get pip3, which is now part of the base system, and
the number of native packages included in the MinGW containers is
significantly reduced.

The corresponding libvirt-ci commit is 4ff697ba0b5d.
Signed-off-by: NAndrea Bolognani <abologna@redhat.com>

Instead of using pre-built containers hosted on Quay, build
containers as part of the GitLab CI pipeline and upload them to the
GitLab container registry for later use.

This will not significantly slow down builds, because containers are
only rebuilt when the corresponding Dockerfile has been modified.
Signed-off-by: NAndrea Bolognani <abologna@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>