Use the container gateway address as the SMTP host

The value 172.17.0.1 is the default gateway on the default Docker
network. However, when using this image with 'docker-compose', a
new network is created, which means that the IP address of the
gateway is different by definition. Because the network is created
at the same time as the containers, the IP address isn't available
when the YAML definition is created.

This approach replaces the static approach with a dynamic approach
executed at first container startup.

start.sh

@@ -67,7 +67,6 @@ if [[ -n "$LSMB_HAVE_DEPRECATED" ]]; then
     echo "!!! DEPRECATED \$SSMTP_* PARAMETERS WILL BE REMOVED in the 1.9 image!!!"
 fi
 
-
 if [[ ! -f ./local/conf/ledgersmb.yaml ]]; then
   cat <<EOF >./local/conf/ledgersmb.yaml
 paths:
@@ -105,6 +104,12 @@ EOF
 
   if [[ -n "$LSMB_MAIL_SMTPHOST" ]]
   then
+      if [[ "$LSMB_MAIL_SMTPHOST" == "__CONTAINER_GATEWAY__" ]]
+      then
+         LSMB_MAIL_SMTPHOST="$(ip route | awk '/default/ { print $3 }')"
+         export LSMB_MAIL_SMTPHOST
+      fi
+
       cat <<EOF >./local/conf/ledgersmb.000.yaml
 mail:
   transport: