Docker reminds me a lot of the PKZIP utilities. For those who don't remember, back in the late 80s the PKZIP utilities became a kind of defacto standard on non Unixes for file compression and decompression. The creator of the utilities was a guy named Phil Katz who meant to make money off of the tools, but as was the fashion at the time released them as basically feature complete shareware.
Some people did register, and quite a few companies registered to maintain compliance so PKWare (the company) did make a bit of money, but most people didn't bother. Eventually the core functionality was simply built into modern Operating Systems and various compatible clones were released for everything under the sun.
Amazingly the company is still around (and even selling PKZIP!) https://www.pkware.com/pkzip
Katz turned out to be a tragic figure http://www.bbsdocumentary.com/library/CONTROVERSY/LAWSUITS/S...
But my point is, I know of many many (MANY) people using Docker in development and deployment and I know of nobody at all who's paying them money. I'm sure they exist, the make revenue from somewhere presumably, but they're basically just critical infrastructure at this point and just becoming an expected part of the OS, not a company.