gitlab: Extract core container jobs to container-core.yml

gitlab: Extract core container jobs to container-core.yml

It is not possible to use the previously extracted templates
without this set of core containers. Extract them into a new
file (container-core.yml) to be able to build them without
having to build all the other containers by default.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Message-Id: <20210519185504.2198573-10-f4bug@amsat.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>