Docker is also free, but the licensing is a bit murkier. For Linux, it is clearly free and open source. For Windows, there are questions, and perhaps only a lawyer can sort it out for us. I will try to learn more, but for now, I don’t have the expertise to make a ruling.

Here is a quote that says explicitly that Docker CE, the one I want is free:

“Docker Engine has been renamed to Docker Community Edition, and, as the name suggests, it’s a do-it-yourself, community supported version of Docker that’s available for free of cost.”

I found that quote here:

https://www.cio.com/article/3176713/open-source-tools/docker-draws-distinctions-between-enterprise-and-community-editions.html

More specifically, it is released under the Apache version 2 license:

https://www.apache.org/licenses/LICENSE-2.0

A key line is this license is here:

Subject to the terms and conditions of this License, each Contributor hereby grants to You a perpetual, worldwide, non-exclusive, no-charge, royalty-free, irrevocable copyright license to reproduce, prepare Derivative Works of, publicly display, publicly perform, sublicense, and distribute the Work and such Derivative Works in Source or Object form.

A link showing that Docker CE uses this license is here:

https://github.com/docker/docker-ce/blob/master/components/cli/LICENSE

Here is the download page for Docker for Windows showing that it is free, but note that there are some caveats which give me pause:

https://hub.docker.com/editions/community/docker-ce-desktop-windows

Here are similar links for Docker:

https://opensource.stackexchange.com/questions/5436/is-docker-still-free-and-open-source

https://www.docker.com/community/open-source

https://www.docker.com/legal/components-licenses