So you are looking at their paid service, but you dont HAVE to use it. Docker is open source and 'free'. It is licensed as open source with Apache.
So software like this, that is open source and 'free', make money by offering a support service.
https://www.docker.com/subscription-servicesHowever, you are able to download Docker and use it
"About the Docker Project
Our open source projects serve as a basis for our free and open Docker platform. In the last two years, over 450 million people worldwide have downloaded Docker’s open source software. This vibrant and ever-growing community of users and contributors represent the spirit the gave birth to, and fuels, Docker the company."->
https://www.docker.com/open-sourcetl:dr, you are legally allowed to use Docker as you wish for free. If you feel like hosting Docker on your own is too much, or you think you need advanced support and security, you can pay for the service.
tl:dr2 -
https://docs.docker.com/engine/faq/Chef offers something similar, and even Jenkins offers a 'paid' service -
https://wiki.jenkins-ci.org/display/JENKINS/Commercial+SupportDownload here for Mac btw ->
https://docs.docker.com/engine/installation/mac/