Add 'DEBIAN_FRONTEND' as 'ARG'

Using 'ARG' instead of 'ENV' allows to define environment variables only
for the build stage. See
https://github.com/moby/moby/issues/4032#issuecomment-34597177.

docker/Dockerfile

@@ -3,6 +3,7 @@ FROM debian:jessie-slim
 LABEL MAINTAINER="theonlydoo <theonlydoo@gmail.com>"
 
 ARG RELEASE=1.6
+ARG DEBIAN_FRONTEND=noninteractive
 
 EXPOSE 5000