The core of docker is just cgroups, and, well, they didn't develop it, that is in the Linux kernel and GPL.
What exactly would an enterprise version offer that would make someone pay a per-server licensing cost?
What exactly would an enterprise version offer that would make someone pay a per-server licensing cost?
Orchestration. Compose/Swarm, which Google gave away for free as k8s (In the interest of completeness, I'll add Hashicorp's Nomad will also orchestrate containers as well as binaries). The idea of Docker monetizing died with that action by Google (and to a lesser extent, Hashicorp, although Nomad does have an enterprise version which corps are paying for and using). Google needed something to slow down AWS adoption, and Docker was collateral damage. The rest of necessary tooling for container management (build, pull, push, local dev management, registries) is fairly trivial [1].
Now anyone can run Kubernetes (or Nomad), either self-hosted or with a hosted provider, which is great for the industry but not so good for Docker.
[1] https://github.com/p8952/bocker (Docker implemented in around 100 lines of bash)
Exactly the same model as Redhat RHEL, who incidentally are the only profitable open source company ever.