bootstrap/.gitlab-ci.yml: add ci file to trigger image auto build

bootstrap/.gitlab-ci.yml: add ci file to trigger image auto build

Depending on 'SAMBA_CI_REBUILD_IMAGES=yes' (and
'SAMBA_CI_REBUILD_BROKEN_IMAGES=yes') as environment
variables on a custom gitlab ci pipeline we'll generate und upload
container images.

bootstrap/README.md will get more details in the next commits.

Please note ci in this file did NOT add the `latest` tag,
since we want main ci to always use a fixed image based on the sha1sum of
everything under bootstrap/ from now on.
This also implies the new built image will not replace/break anything.

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>