Guestbook build process employs the usage of docker-in-docker to build an image within another. This requires that the build image has access to the `docker` program's binary, which defaults to the docker available on your host machine. In the case of boot2docker, `DOCKER_BIN` must be set to the binary's location in the boot2docker's vm.
Releasing the image requires that you have access to the docker registry user account which will host the image.