I'm surprised more people aren't mentioning RancherOS. It's a great "Docker Only" OS, like many of the other options mentioned. It doesn't have the market that CoreOS did, that's for sure, but still a great tool. It's tiny and being actively updated.
Plus it's got good RPi support for all those serious Big Software Company deployments /s
Plus it's got good RPi support for all those serious Big Software Company deployments /s