Which makes me wonder, would docker have gained traction if they didn't offer free registry services?

But there's a reason why Docker was so successful - a single file that could define a deployment and tooling to build it into a runnable artefact was incredibly useful. From the future tech useful.

If you can name these other similar solutions created by charities I can probably me more specific.

I think of Docker as a well executed and well-timed formalization of existing tools. It put a name to a collection of engineering concepts. This is why people can build Docker in N lines of shell script. Jails existed in the late 90s. IaC had multiple options before Docker.