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"]

postfix/docker-compose.yml

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+version: '3'
+services:
+  postfix:
+    image: postfix:latest
+    container_name: postfix
+    restart: always
+    networks:
+      - kopano
+    volumes:
+      - ~/kopano/postfix/etc:/etc/postfix
+      - spool:/var/spool/postfix
+networks:
+  - kopano
+volumes:
+  - spool: