Getting exit code 3 from slugbuilder

gistfile1.sh

# Cat tar contents into container
docker@boot2docker:~$ id=$(cat source.tar | docker run -i -a stdin flynn/slugbuilder)
docker@boot2docker:~$ docker wait $id
3
docker@boot2docker:~$ docker logs $id
-----> Ruby app detected
-----> Compiling Ruby/Rails
...
-----> Compiled slug size is 71M



# Mount volume and run from inside container
docker@boot2docker:~$ ls
boot2docker, please format-me  source.tar
docker@boot2docker:~$ docker run -ti -v /home/docker/:/var/source/ --entrypoint bash flynn/slugbuilder -s
slugbuilder@27b03669cb52:/$ cat /var/source/source.tar | /tmp/builder/build.sh
-----> Ruby app detected
-----> Compiling Ruby/Rails
...
-----> Compiled slug size is 71M
slugbuilder@27b03669cb52:/$ echo $?
0