As suggested by Benjamin, this merely runs a container-build job off
the base image without any "content". The order (for Arch) is thus:

arch:latest@container-build creates an image with curl+wget installed
and pushes that to the registry.

arch:latest@base-image starts with that base image but doesn't install
anything. The DISTRO_EXEC script runs curl and wget though - which we
expect to be present since we have our custom base image. If that
succeeds we don't need to do anything else, we expect the rest to work
since it's just the container-build job.

The only unfortunate hack is that we cannot use the global
FDO_DISTRIBUTION_TAG to reference the image. Gitlab doesn't do
double-expansion of variables so we need to hardcode the same value.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>