Always choose the default IPv4 address for init'ing

Closes coop-cloud/abra#91.

abra

@@ -1509,7 +1509,10 @@ sub_server_list() {
 help_server_init (){
   echo "abra [options] server init
 
-Set up a server for Docker swarm joy.
+Set up a server for Docker swarm joy. This initialisation explicitly chooses
+for the \"single host swarm\" mode which uses the default IPv4 address as the
+advertising address. This can be re-configured later for more advanced use
+cases.
 
 POWERED BY
   docker swarm init
@@ -1521,7 +1524,12 @@ sub_server_init() {
 
   load_context
 
-  docker swarm init || true
+  # Note(decentral1se): it sucks to use Google DNS but seems like a reliable method
+  #                     for determining the default IPv4 address especially nowadays
+  #                     when there are often multiple internal addresses assigned to eth0
+  default_ipv4="$(ip route get 8.8.8.8 | head -1 | awk '{print $7}')"
+
+  docker swarm init --advertise-addr "$default_ipv4" || true
   docker network create --driver=overlay proxy --scope swarm || true
 }