StraightDocker
There's no source code here; it's just a place to store Windows binary releases of Docker.
The main Docker project (moby/moby) releases nightly binaries at https://master.dockerproject.org. However, only the very latest build is available, generally (there are some ancient builds there; I don't know why). I'm using this project to host a stable release point of known good builds of the Windows binaries.
A release hosted here may be simply a copy of a master.dockerproject.org nightly build, or it may consist of privately-built binaries, built from some other branch (such as to pick up new features and/or bugfixes before they get merged into Moby's master branch, which can sometimes take a while). The description of a release will tell you what is in it.
I use the term "straight Docker" to differentiate between "Docker for Windows" (D4W), which is a closed-source product made by Docker, Inc; and the open-source components from the Moby project ("straight Docker") which D4W builds upon.