As other mentioned before. Docker the core product is open source and free but in order to use Docker in production you need many supporting products/features.

As other mentioned before. Docker the core product is open source and free but in order to use Docker in production you need many supporting products/features. Beyond the support which is an obvious path, they have the paid private hosting, the cloud hosting and more. If you look at the ecosystem of orchestration and deployment of docker you see that there's still HUGE room for improvement and people WILL pay for a good product in this area.