It is a tool written in php for learning purpose only for students and researchers.
For students who are trying to learn , my advise would be to go to Reflected section first then dom then stored.
For Researchers the Custom section in Reflected section will provide you to construct your own injection environment in a simple way. The document is pretty much self explanatory.
For docker installation, Run:
docker pull hellboyb2/agnikund
To run this server go into the AgniKund-XSS_Playground folder and run the command
php -S localhost:8000 -t ./ ./route.php