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:
More specifically, it is released under the Apache version 2 license:
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:
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:
Here are similar links for Docker: