Do better at finding ssh_info for Docker

plugins/providers/docker/provider.rb

@@ -136,16 +136,18 @@ module VagrantPlugins
         # If the container isn't running, we can't SSH into it
         return nil if state.id != :running
 
-        network = driver.inspect_container(@machine.id)['NetworkSettings']
-        ip      = network['IPAddress']
+        port_name = "#{@machine.config.ssh.guest_port}/tcp"
+
+        network   = driver.inspect_container(@machine.id)['NetworkSettings']
+        port_info = network['Ports'][port_name].first
 
         # If we were not able to identify the container's IP, we return nil
         # here and we let Vagrant core deal with it ;)
-        return nil if !ip
+        return nil if port_info.nil? || port_info.empty?
 
         {
-          host: ip,
-          port: @machine.config.ssh.guest_port
+          host: port_info['HostIp'],
+          port: port_info['HostPort']
         }
       end