Get rid of hard coded IPs (#441)

Replace them with:
* containers' hostnames
* gateway's IP address for default SMTP_HOST
* Core: when SMTP_HOST undef, echo the default value
* docker-compose: add port mapping
* docker-compose: drop network driver conf
* Exim macro: use localhost as LMTP host
* Update README.md
* docker-compose: Limit port mapping to loopback
* Update other docker-compose example files

core/docker-entrypoint.sh

@@ -27,14 +27,16 @@ function wait_for_mysql () {
 echo "# This file is autogenerated at container startup." > /etc/mailman.cfg
 
 # Check if $MM_HOSTNAME is set, if not, set it to the value returned by
-# `hostname -i` command to set it to whatever IP address is assigned to the
+# `hostname` command to set it to whatever hostname is assigned to the
 # container.
 if [[ ! -v MM_HOSTNAME ]]; then
-	export MM_HOSTNAME=`hostname -i`
+	export MM_HOSTNAME=`hostname`
 fi
 
+# SMTP_HOST defaults to the gateway
 if [[ ! -v SMTP_HOST ]]; then
-	export SMTP_HOST='172.19.199.1'
+	export SMTP_HOST=$(/sbin/ip route | awk '/default/ { print $3 }')
+	echo "SMPT_HOST not specified, using the gateway ($SMTP_HOST) as default"
 fi
 
 if [[ ! -v SMTP_PORT ]]; then