.gitlab-ci.yml: make use of bootstrap/.gitlab-ci.yml and use the new defined image

.gitlab-ci.yml: make use of bootstrap/.gitlab-ci.yml and use the new defined image

See bootstrap/README.md for the instructions to create and upload the
images via a custom gitlab ci pipeline.

The key is that it's always possible to regenerate the image if
it's not present in the container registry, where we are free to
delete old images. But it should be possible to rebuild images
if someone has the need to run a pipeline based on an old
branch.

Pair-Programmed-With: Stefan Metzmacher <metze@samba.org>

Signed-off-by: Joe Guo <joeg@catalyst.net.nz>
Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>