Choosing the correct worker for a project

I can't find a way to map a project to specific worker or worker types. Is this on the TODO list?
I see that [travisyml.py](https://github.com/buildbot/buildbot_travis/blob/master/buildbot_travis/travisyml.py#L49) has an image attribute, but it's currently not used as far as I can see? Is this meant to handle this in the future?

Start by reading buildbot_travis/travisyml.py around line 49, including how the image attribute is parsed and used. Trace the project and worker configuration paths, then clarify the intended mapping behavior; the issue is complete only when that behavior and its expected configuration are defined and implemented.

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