Fix --network-add adding duplicate networks

When adding a network using `docker service update --network-add`,
the new network was added by _name_.

Existing entries in a service spec are listed by network ID, which
resulted in the CLI not detecting duplicate entries for the same
network.

This patch changes the behavior to always use the network-ID,
so that duplicate entries are correctly caught.

Before this change;

    $ docker network create -d overlay foo
    $ docker service create --name=test --network=foo nginx:alpine
    $ docker service update --network-add foo test
    $ docker service inspect --format '{{ json .Spec.TaskTemplate.Networks}}' test
    [
      {
        "Target": "9ot0ieagg5xv1gxd85m7y33eq"
      },
      {
        "Target": "9ot0ieagg5xv1gxd85m7y33eq"
      }
    ]

After this change:

    $ docker network create -d overlay foo
    $ docker service create --name=test --network=foo nginx:alpine
    $ docker service update --network-add foo test
    service is already attached to network 9ot0ieagg5xv1gxd85m7y33eq

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cli/command/service/opts.go

@@ -357,12 +357,12 @@ func convertNetworks(ctx context.Context, apiClient client.NetworkAPIClient, net
 	var netAttach []swarm.NetworkAttachmentConfig
 	for _, net := range networks.Value() {
 		networkIDOrName := net.Target
-		_, err := apiClient.NetworkInspect(ctx, networkIDOrName, types.NetworkInspectOptions{Scope: "swarm"})
+		nw, err := apiClient.NetworkInspect(ctx, networkIDOrName, types.NetworkInspectOptions{Scope: "swarm"})
 		if err != nil {
 			return nil, err
 		}
 		netAttach = append(netAttach, swarm.NetworkAttachmentConfig{
-			Target:     net.Target,
+			Target:     nw.ID,
 			Aliases:    net.Aliases,
 			DriverOpts: net.DriverOpts,
 		})