gitlab-ci: introduce containers with IGT inside

Currently, the containers are used only to build IGT. We also want to
have an option to run IGT inside a container. For this, new build stage
is added.

v2: help touch-ups (Arek)

Reviewed-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Signed-off-by: Oleg Vasilev <oleg.vasilev@intel.com>