Usage is quite simple:

git clone https://github.com/FluidTYPO3/fluidtypo3-docker.git
cd fluidtypo3-docker
docker build -t fluidtypo3 .
docker-compose up

Performing these steps will start a default FluidTYPO3 site ready to be unrolled using EXT:site. To perform the final steps, log in to the TYPO3 backend, go to the "Extension Manager", locate the extension called "FluidTYPO3 Site Kickstarter", click the little gear icon to enter extension settings and fill these settings with the configuration you desire. Finally, save the configuration view and EXT:site will unroll pages, templates etc. and remove itself.