Update docker.rb for Podman

This if statement will enable the fact to work if you are using podman with docker symlink/support as the json syntax is lowercase on podman but camel/title case on docker.

As per - puppetlabs#982

lib/facter/docker.rb

@@ -138,7 +138,11 @@ def interfaces
         docker_network_names.each do |network|
           inspect = JSON.parse(Facter::Core::Execution.execute("#{docker_command} network inspect #{network}", timeout: 90))
           docker['network'][network] = inspect[0]
-          network_id = docker['network'][network]['Id'][0..11]
+          if docker['network'][network]['Id'].nil?
+            network_id = docker['network'][network]['id'][0..11]
+       	  else
+            network_id = docker['network'][network]['Id'][0..11]
+       	  end
           interfaces.each do |iface|
             docker['network']['managed_interfaces'][iface] = network if %r{#{network_id}}.match?(iface)
           end