Add more error handling and refactoring
Link docker-enter as well

docker-attach may be replaced with using nsinit
Fix an error on building a box

- rm: can't remove '/var/lib/docker/aufs': Device or resource busy
- rm: can't remove '/var/lib/docker': Directory not empty
Correct the default command
Fix a bug in the docker-attach and rename it docker-enter
Revise tests for docker-enter
Revise README for Parallels