Dockerfile

FROM ubuntu:xenial

ENV DEBIAN_FRONTEND noninteractive

RUN rm -rf /var/lib/apt/lists/* && \
    apt-get update -q -q && \
    echo 'UTC' > /etc/timezone && \
    dpkg-reconfigure tzdata

COPY postfix_3.0.4-5ubuntu1_amd64.deb /root/
RUN apt-get -y install libicu55 libsasl2-2 libsqlite3-0 libssl1.0.0 \
    netbase ssl-cert cpio
RUN dpkg -i /root/postfix_3.0.4-5ubuntu1_amd64.deb && \
    rm /root/postfix_3.0.4-5ubuntu1_amd64.deb
RUN postconf smtputf8_enable=yes && \
    postconf smtpd_use_tls=no && \
    postconf smtpd_banner='$myhostname ESMTP $mail_name (EAI)' && \
    postconf virtual_mailbox_base=/var/mail/vhosts && \
    true
RUN apt-get -y install rsyslog

VOLUME /etc/postfix/tables
VOLUME /var/mail/vhosts
EXPOSE 25

RUN rm -rf /var/lib/apt/lists/*

COPY undaemonize.sh /root/
CMD /bin/bash root/undaemonize.sh

undaemonize.sh

#! /usr/bin/env bash
set -eu

# Env. vars
VTABLE=${VTABLE:-/etc/postfix/tables/virtual}
USERID=${USERID:-5000}
GROUPID=${GROUPID:-5000}

# Setup
ln -sf "$VTABLE" /etc/postfix/virtual
postconf virtual_mailbox_maps=hash:/etc/postfix/virtual
postconf virtual_uid_maps=static:$USERID
postconf virtual_gid_maps=static:$GROUPID
postmap -op hash:/etc/postfix/virtual
/etc/init.d/rsyslog start

pidfile="/var/spool/postfix/pid/master.pid"
postfix="/usr/sbin/postfix"

# Proxy signals
function kill_app() {
    $postfix stop
    exit 0 # exit okay
}

trap "kill_app" SIGINT SIGTERM

# Launch daemon
$postfix start -w

# Show the log, following through file changes
tail -F /var/log/mail.log

exit 3