set ENV TERM and ENV DEBIAN_FRONTEND to prevent useless error

More info can be found in [docker issue #4032](moby/moby#4032 (comment))
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

Author: Édouard Lopez

Dockerfile

@@ -1,5 +1,8 @@
 FROM debian:wheezy-backports
 
+ARG TERM=linux
+ARG DEBIAN_FRONTEND=noninteractive
+
 # restyaboard version
 ENV restyaboard_version=v0.2.1