pluginMain: remove uses of DockerCLI.Apply

The Apply method was added when CLI options for constructing the CLI were
rewritten into functional options in [cli@7f207f3]. There was no mention
in the pull request of this method specifically, and we want to remove or
reduce functions that mutate the CLI configuration after initialization
if possible (and likely remove the `Apply` method).

This patch removes the use of the `Apply` function as an intermediate step;
improvements will be made in the CLI itself for a more solid implementation.

[cli@7f207f3]: https://github.com/docker/cli/commit/7f207f3f957ed3f5129aeb22bef2a429c14caf22

Signed-off-by: Sebastiaan van Stijn <[email protected]>

cmd/main.go

@@ -47,8 +47,15 @@ func pluginMain() {
 			if err := plugin.PersistentPreRunE(cmd, args); err != nil {
 				return err
 			}
-			// compose-specific initialization
-			dockerCliPostInitialize(dockerCli)
+			// compose-specific initialization. This must be called AFTER
+			// plugin.PersistentPreRunE.
+			//
+			// FIXME(milas): remove once https://github.com/docker/cli/pull/4574 is merged; for now,
+			//  set it in a rather roundabout way by grabbing the underlying
+			//  concrete client and manually invoking an option on it
+			if mobyClient, ok := dockerCli.Client().(*client.Client); ok {
+				_ = client.WithUserAgent("compose/" + internal.Version)(mobyClient)
+			}
 
 			if err := cmdtrace.Setup(cmd, dockerCli, os.Args[1:]); err != nil {
 				logrus.Debugf("failed to enable tracing: %v", err)
@@ -75,22 +82,6 @@ func pluginMain() {
 		})
 }
 
-// dockerCliPostInitialize performs Compose-specific configuration for the
-// command.Cli instance provided by the plugin.Run() initialization.
-//
-// NOTE: This must be called AFTER plugin.PersistentPreRunE.
-func dockerCliPostInitialize(dockerCli command.Cli) {
-	// HACK(milas): remove once docker/cli#4574 is merged; for now,
-	// set it in a rather roundabout way by grabbing the underlying
-	// concrete client and manually invoking an option on it
-	_ = dockerCli.Apply(func(cli *command.DockerCli) error {
-		if mobyClient, ok := cli.Client().(*client.Client); ok {
-			_ = client.WithUserAgent("compose/" + internal.Version)(mobyClient)
-		}
-		return nil
-	})
-}
-
 func main() {
 	if plugin.RunningStandalone() {
 		os.Args = append([]string{"docker"}, compatibility.Convert(os.Args[1:])...)