Umm. There isn't really source code. All I did was take my Raspbian minimal image, mount this as a directory, then used the docker commands to create an image from this directory.
Docker is pretty secure anyway. Even if there was something malicious, it can't touch the host OS. That's one of the things that makes it great.
Could you provide a github link to a build environment?
This image is great ... but I only do trusted builds ...