Few stack deploy network fixes

- Make sure we use the correct network name for external ones.
- Make the default network overridable and only creates networks that
  are used by services — so that default network is only created if a
  service doesn't declare a network.

Signed-off-by: Vincent Demeester <vincent@sbr.pm>

command/stack/deploy.go

@@ -117,7 +117,9 @@ func deployCompose(ctx context.Context, dockerCli *command.DockerCli, opts deplo
 
 	namespace := convert.NewNamespace(opts.namespace)
 
-	networks, externalNetworks := convert.Networks(namespace, config.Networks)
+	serviceNetworks := getServicesDeclaredNetworks(config.Services)
+
+	networks, externalNetworks := convert.Networks(namespace, config.Networks, serviceNetworks)
 	if err := validateExternalNetworks(ctx, dockerCli, externalNetworks); err != nil {
 		return err
 	}
@@ -131,6 +133,20 @@ func deployCompose(ctx context.Context, dockerCli *command.DockerCli, opts deplo
 	return deployServices(ctx, dockerCli, services, namespace, opts.sendRegistryAuth)
 }
 
+func getServicesDeclaredNetworks(serviceConfigs []composetypes.ServiceConfig) map[string]struct{} {
+	serviceNetworks := map[string]struct{}{}
+	for _, serviceConfig := range serviceConfigs {
+		if len(serviceConfig.Networks) == 0 {
+			serviceNetworks["default"] = struct{}{}
+			continue
+		}
+		for network := range serviceConfig.Networks {
+			serviceNetworks[network] = struct{}{}
+		}
+	}
+	return serviceNetworks
+}
+
 func propertyWarnings(properties map[string]string) string {
 	var msgs []string
 	for name, description := range properties {