Dockerfile: use DEBIAN_FRONTEND=noninteractive as ARG ie just on build

Dockerfile

@@ -1,10 +1,11 @@
 FROM ubuntu:18.04
 WORKDIR /ardupilot
 
+ARG DEBIAN_FRONTEND=noninteractive
 RUN useradd -U -m ardupilot && \
     usermod -G users ardupilot
 
-RUN DEBIAN_FRONTEND=noninteractive apt-get update && apt-get install --no-install-recommends -y \
+RUN apt-get update && apt-get install --no-install-recommends -y \
     lsb-release \
     sudo \
     software-properties-common
@@ -35,7 +36,7 @@ RUN echo "if [ -d \"\$HOME/.local/bin\" ] ; then\nPATH=\"\$HOME/.local/bin:\$PAT
 ENV BUILDLOGS=/tmp/buildlogs
 
 # Cleanup
-RUN DEBIAN_FRONTEND=noninteractive sudo apt-get clean \
+RUN sudo apt-get clean \
     && sudo rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/*
 
 ENV CCACHE_MAXSIZE=1G