separated components

postfix/Dockerfile

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+# syntax=docker/dockerfile:1.3-labs
+FROM ubuntu:20.04
+# install apt packages
+RUN <<EOF 
+apt-get update -y
+export DEBIAN_FRONTEND=noninteractive 
+export TZ=Europe/Berlin 
+apt-get install -y tzdata postfix spampd wget xz-utils
+apt-get autoclean
+rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/* ~/.cache ~/.npm
+EOF
+WORKDIR /root
+EXPOSE 25
+VOLUME /var/spool/postfix
+ENTRYPOINT ["/usr/sbin/postmulti -i %i -p start"]