Implementation of multi-domain relay hosts (#922, #926)

* Add new configuration for multi-domain relay hosts (#922)
 * Creates new environment variables (replacing existing AWS_SES variables)
 * Optionally allows more advanced setups using config files
* Update relay hosts during change detection (#922)
* Add helper scripts for adding relay hosts and per-domain auth
* Allow the possibility to deliver some mail directly
* adding a domain with no destination will exclude it from the
  relayhost_map and so Postfix will attempt to deliver the mail directly
* tests for setup.sh script
* tests for relay host configuration
* these tests cover the code in `start-mailserver.sh` dealing with both
  the env vars and the configuration files

target/bin/addrelayhost

@@ -0,0 +1,33 @@
+#! /bin/bash
+
+DATABASE=${DATABASE:-/tmp/docker-mailserver/postfix-relaymap.cf}
+
+DOMAIN="$1"
+HOST="$2"
+PORT="$3"
+
+usage() {
+	echo "Usage: addrelayhost <domain> <host> [<port>]"
+}
+
+errex() {
+	echo "$@" 1>&2
+	exit 1
+}
+
+escape() {
+	echo "${1//./\\.}"
+}
+
+[ -z "$DOMAIN" ] && { usage; errex "no domain specified"; }
+[ -z "$HOST" ] && { usage; errex "no relay host specified"; }
+
+if [ -z "$PORT" ]; then
+	PORT=25
+fi
+
+if grep -qi "^@$DOMAIN" $DATABASE 2>/dev/null; then
+	sed -i "s ^@"$DOMAIN".* "@$DOMAIN"\t\t["$HOST"]:"$PORT" " $DATABASE
+else
+	echo -e "@$DOMAIN\t\t[$HOST]:$PORT" >> $DATABASE
+fi