feat: Replace old bytemark/smtp image with the source code

This will allow more platforms to use this template repo. The original
dockerhub image was last updated 5 years ago, and as such,
newer architectures aren't supported like the Raspberry Pi 4
which a lot of people are using for self hosting these days

smtp/stretch/Dockerfile

@@ -0,0 +1,13 @@
+FROM debian:stretch-slim
+
+# Install exim4
+ENV DEBIAN_FRONTEND noninteractive
+RUN set -ex; \
+    apt-get update; \
+    apt-get install -y exim4-daemon-light; \
+    apt-get clean
+
+COPY docker-entrypoint.sh /usr/local/bin/docker-entrypoint.sh
+EXPOSE 25/tcp
+ENTRYPOINT [ "docker-entrypoint.sh" ]
+CMD [ "exim", "-bdf", "-v", "-q30m" ]

smtp/stretch/docker-entrypoint.sh

@@ -0,0 +1,53 @@
+#!/bin/sh
+set -e
+CONFDIR=/etc/exim4
+
+# By default, send email directly to the recipient.
+DC_EXIMCONFIG_CONFIGTYPE="internet"
+
+# By default, only hosts on the private network can use the smart host (ie,
+# only other containers, not the whole internet); a thin layer of protection
+# in case port 25 is accidentally exposed to the public internet.
+DC_RELAY_NETS="10.0.0.0/8;172.16.0.0/12;192.168.0.0/16"
+
+# If RELAY_HOST has been set then switch to smart host configuration.
+if [ "x$RELAY_HOST" != "x" ]; then
+    DC_EXIMCONFIG_CONFIGTYPE="satellite"
+    DC_SMARTHOST="$RELAY_HOST::${RELAY_PORT:-25}"
+    if [ "x$RELAY_USERNAME" != "x" ] && [ "x$RELAY_PASSWORD" != "x" ]; then
+        printf '%s\n' "*:$RELAY_USERNAME:$RELAY_PASSWORD" > "$CONFDIR/passwd.client"
+    fi
+fi
+
+# Set which hosts can use the smart host.
+if [ "x$RELAY_NETS" != "x" ]; then
+    DC_RELAY_NETS="$RELAY_NETS"
+fi
+
+# Write exim configuration.
+cat << EOF > "$CONFDIR/update-exim4.conf.conf"
+dc_eximconfig_configtype='$DC_EXIMCONFIG_CONFIGTYPE'
+dc_other_hostnames=''
+dc_local_interfaces=''
+dc_readhost=''
+dc_relay_domains=''
+dc_minimaldns='false'
+dc_relay_nets='$DC_RELAY_NETS'
+dc_smarthost='${DC_SMARTHOST:-}'
+CFILEMODE='644'
+dc_use_split_config='false'
+dc_hide_mailname='true'
+dc_mailname_in_oh='true'
+dc_localdelivery='mail_spool'
+EOF
+
+# Set primary_hostname.
+if [ "x$MAILNAME" != "x" ]; then
+    printf '%s\n' "$MAILNAME" > /etc/mailname
+    printf '%s\n' "MAIN_HARDCODE_PRIMARY_HOSTNAME=$MAILNAME" >> "$CONFDIR/update-exim4.conf.conf"
+fi
+
+# Apply exim configuration.
+update-exim4.conf
+
+exec "$@"