CoreOS, docker, and mesos was an alternative to kubernetes. I'd say while using flatcar is viable, it looks like kubernetes is what the people have chosen.
Me, too! I got started with both CoreOS and kubernetes using their kubernetes-in-vagrant repo (before it went to the abysmal rkt runtime:
https://github.com/coreos/coreos-kubernetes/blob/v0.4.0/mult... ) and I shepherded that v1.0.6 cluster all the way up to v1.10 when I left. It was a highly educational process
I really prefer DC/OS to K8s. They both have strengths and weaknesses, but I hate we're not this k8s monoculture now. There need to be more container orchestration systems .. and don't kid yourself; few shops are using nomad.
nomad, k8s, mesos+marathon (or something plus mesos), traditional deployment
That's probably about it. Right now, I'm jumping on the k8s bandwagon, but looking to keep it simple. As much as I'd like to look at nomad seriously, too many times I've been caught by not looking at the mainstream.