Expand on docker-setup.sample script so it creates docker network "kamal" on each of the defined hosts.

lib/kamal/cli/templates/sample_hooks/docker-setup.sample

@@ -1,7 +1,13 @@
-#!/bin/sh
+#!/usr/bin/env ruby
 
 # A sample docker-setup hook
 #
-# Sets up a Docker network which can then be used by the application’s containers
+# Sets up a Docker network on defined hosts which can then be used by the application’s containers
 
-ssh user@example.com docker network create kamal
+hosts = ENV["KAMAL_HOSTS"].split(",")
+
+hosts.each do |ip|
+  destination = "root@#{ip}"
+  puts "Creating a Docker network \"kamal\" on #{destination}"
+  `ssh #{destination} docker network create kamal`
+end