enable byobu in defult and fix
enable byobu in defult and fix

- enable byobu in default by 'byobu-enable'
- fix missing RUN
- add comments
- reduce docker stage

Signed-off-by: Hiroshi Miura <ff6fb3ed059786151e736af13627c08d24e9f922@linux.com>

miurahr/docker-pyvenv-dev,miurahr/docker-pyvenv