There's Docker images there. Including for 1.5.5 and 1.6.4.
But they're not always up-to-date - 1.5.6 and 1.6.5 should be available.
For (I think) the second time in as many months, a security update to RoundCube has been released but the Docker images were not updated, leaving those using them vulnerable. How are users of the official Docker images supposed to update when the images don't get updated in a timely manner?