CI: Allow running in a container.

CI: Allow running in a container.

Add a parameter of the pr-gate-build-job template to specify a
container image URL.  If the value is not '' (default), then the
jobs will be run inside a container based on that image.

Signed-off-by: Oliver Steffen <osteffen@redhat.com>
Reviewed-by: Michael Kubacki <michael.kubacki@microsoft.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Chris Fernald <chfernal@microsoft.com>