gitlab-ci: Update to current templates
The .fdo.container-ifnot-exists template has been replaced by .fdo.container-build.
We need to include "debian/" in FDO_REPO_SUFFIX for now, we can drop it for individual images when their tags are bumped if we want.
Miscellaneous other goodies this gets us:
- The templates now add some labels to images which may be useful for garbage collecting unused tags in the future.
- The templates now copy the current tag from the main project registry to the forked project's if it already exists in the latter but points to a different image hash. This will avoid false failures (or passes) due to using the wrong image.