But the Docker engine (that runs containers on the local machine) is only available on Mac and Windows via Docker Desktop which is not free-as-in-beer anymore for all.
This is incorrect. The Docker engine shipped in Docker for Mac is built from the exact same Docker Engine in the open-source release. It’s the native Mac application wrapping the engine in a single-purpose hypervisor that is closed source.
Why? Anyone can assemble an equivalent from available open-source tools:
* Virtualbox
* Your favorite Linux distro
* Docker engine on the Linux VM
* Docker CLI on the Mac host
* A variety of filesystem sync solutions (I don’t remember their names but there are several)
Alternatively there’s also docker-machine.
The closed source app gets you the convenience of not having to set that all up. If you don’t like installing closed source apps you probably prefer to set things up yourself anyway. So what’s the problem?