Boxer started its life as a sort of frontend but is now a fork of DOSBox. All of the changes are meant to fix (replace) the awful workflow that DOSBox currently has with mounting drives and tweaking config files and etc.

DOSBox is mentioned in the About window, the credits, and Boxer itself when you launch a gamebox. It also used to be on the website, though checking now I'm not sure where it's gone.

Boxer is properly GPL and you can find the source code here:

https://bitbucket.org/alunbestor/boxer