gitlab: add special rule for the hexagon container

gitlab: add special rule for the hexagon container

The hexagon container is always manually built but of course not
everyone will be building it themselves and pushing to their
registries. We still need to create a "local" registry copy for the
actual gitlab tests to run. We don't build it in this case, just pull
it across from the upstream registry. We disable this rule from
running on the qemu-project itself so it doesn't accidentally wipe out
our master copy.

Fixes: 910c40ee94 ("gitlab: add build-user-hexagon test")
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Cc: Cornelia Huck <cohuck@redhat.com>
Message-Id: <20210520174303.12310-4-alex.bennee@linaro.org>