- Remove temporary files and APT's cache, reduce image size.

Dockerfile

 FROM debian:jessie
 ENV DEBIAN_FRONTEND noninteractive
 RUN apt-get update && \
-    apt-get install -y unbound unbound-host dnsutils dma bsd-mailx
+    apt-get install -y unbound unbound-host dnsutils dma bsd-mailx && \
+    rm -rf /tmp/* /var/tmp/* /var/lib/apt/lists/* /var/cache/apt/archives/*
 COPY entrypoint.sh /entrypoint.sh
 ENTRYPOINT ["/entrypoint.sh"]
 CMD ["/bin/bash", "--login"]