> 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)

As opposed to K8 implemented in around 100 million lines of whatever.

Security features, support, integration with a Docker Data Centre type management console and other complementary tools.

Exactly the same model as Redhat RHEL, who incidentally are the only profitable open source company ever.

There are a bunch of profitable open source companies (SUSE, nginx, Joyent, etc) -- Red Hat just happens to be largest.

Hortonworks was profitable before Cloudera acquisition. Rob Bearden was CEO of Hortonworks before coming to Docker as well.

Given Docker's a packaging format (effectively) a packaging and distribution story that's at least on a par with Debian or Red Hat would be valuable. Instead of rediscovering packing and distribution from scratch and delivering complete sacks of shit like "30% of images on dockerhub have serious security vulnerabilities" or "whoops leaked our user credentials".

Nothing. Without any secret sauce, an open source clone of an "enterprise" Docker would have been built virtually overnight.

Docker does have an "enterprise" version (called Docker EE). I'm surprised so few people have heard of it.

Probably because there are so few reasons to use it.

A much better alternative is Rancher. 100% open source.