Things has changed a little few days ago. They've sent an email telling their user about their new policies including a "new inactive image retention policy", saying "For free accounts, Docker offers free retention of images inactive for six months" (See link 1).

Not too big of a deal, but I'd imagine many users would take that into account when they're making chooses.

Link 1: https://www.docker.com/blog/scaling-dockers-business-to-serv...