Published releases get pushed when they are ready 
Unfortunately the release tracking is internal, so it's not something you can query.
You can always use the containous/traefik image, with the experimental tags, that are built off the head of each release branch:
containous/traefik:experimental-v2.0 will give you the most recent build on the 2.0 release.
Please be aware that this is a development image, and may be subject to change, and is not guaranteed to work, or be functional, as it is not a release image.