Automated Docker Hub Build Mounted Source Directory?

We are trying to set up automatic builds of a Docker image using Docker Hub. We noticed that one of the first things that happens is it clones the source repository with the docker file – does this source directory have a volume that gets mounted during the docker build of the docker file so that the source checkout can be reused? Is there a way to get this directory mounted for use in the docker build? Or is the “best practice” to git clone another copy of the source code inside the dockerfile?