feat: add DEBIAN_FRONTEND=noninteractive

For common and build only.  Changes back to interactive at the end of dev for end-user (developers) convenience.

Based on discussions with KemoNine.

Dockerfile

@@ -2,6 +2,8 @@ FROM debian:stable-20201117-slim AS common
 
 CMD ["/bin/bash"]
 
+ENV DEBIAN_FRONTEND=noninteractive
+
 ARG REPOSITORY_URL=https://github.com/innovaker/zmk-docker
 LABEL org.opencontainers.image.source ${REPOSITORY_URL}
 
@@ -55,6 +57,8 @@ RUN \
   && apt-get clean \
   && rm -rf /var/lib/apt/lists/*
 
+ENV DEBIAN_FRONTEND=
+
 #------------------------------------------------------------------------------
 
 FROM common AS build