feat: add Internal property to Network struct

[pini-gh]

The purpose of this pull request is to add the Internal property to the Network structure so that it could be used to fix nginx-proxy/nginx-proxy#2310.

I've tested that this change makes the fix for the nginx-proxy related issue easy:

diff --git a/nginx.tmpl b/nginx.tmpl
index fb0766b..afe5c96 100644
--- a/nginx.tmpl
+++ b/nginx.tmpl
@@ -75,7 +75,7 @@
                 {{- continue }}
             {{- end }}
             {{- range sortObjectsByKeysAsc $.globals.CurrentContainer.Networks "Name" }}
-                {{- if and . .Gateway }}
+                {{- if and . .Gateway (not .Internal) }}
     #         container is in host network mode, using {{ .Name }} gateway IP
                     {{- $ip = .Gateway }}
                     {{- break }}

I have no knowledge in Go programming and it took me quite some time to come up with this proposal. Please bear with me :)

[buchdag]

Nice !