set ARG TERM and ARG DEBIAN_FRONTEND to prevent useless error

update to v0.2.1

More info can be found in [docker issue #4032](https://github.com/docker/docker/issues/4032#issuecomment-46128765)
Remove errors such as:
> debconf: unable to initialize frontend: Dialog
> debconf: (TERM is not set, so the dialog frontend is not usable.)
> debconf: falling back to frontend: Readline
> debconf: unable to initialize frontend: Readline
> debconf: (This frontend requires a controlling tty.)
> debconf: falling back to frontend: Teletype

leverage docker caching mechanism and improve readability

Because occured db access denied.

for maintenance

similar issue
https://github.com/docker/docker/issues/6980

and reduce docker image size

and fixed README.md.

install postfix