Is Docker on Windows Servers "free"?
Looks like it does take Docker Enterprise, but that Microsoft includes a license for that in Windows Server.
You can install it with these powershell commands:
Install-Module -Name DockerMsftProvider -Repository PSGallery -ForceInstall-Package -Name docker -ProviderName DockerMsftProvider -ForceStart-Service docker https://blog.sixeyed.com/getting-started-with-docker-on-windows-server-2019/
If you have Symantec Antivirus, you will need to disable it before the Install-Package command will work (or you get a weird "dummy" folder error).