Use ARG instead of ENV for DEBIAN_FRONTEND in Dockerfile

The ENV instruction will set the environment variable in build process & when the container is running. https://docs.docker.com/engine/reference/builder/#env

ENV DEBIAN_FRONTEND noninteractive

The ARG will set the variable only in build process (docker build) and not when the container is running (docker run). as this variable is probably not needed when the container is running, change the ENV to ARG.