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

web/docker-entrypoint.sh

@@ -53,6 +53,11 @@ function check_or_create () {
 # END
 # }
 
+# SMTP_HOST defaults to the gateway
+if [[ ! -v SMTP_HOST ]]; then
+	export SMTP_HOST=$(/sbin/ip route | awk '/default/ { print $3 }')
+fi
+
 # Check if $SECRET_KEY is defined, if not, bail out.
 if [[ ! -v SECRET_KEY ]]; then
 	echo "SECRET_KEY is not defined. Aborting."