feat(cli): report more useful User-Agent on engine API requests

When using the Moby/Docker Engine API client, we do not have a
useful user agent value being reported. Ideally, in the future,
the Docker CLI will set this appropriately for plugins when it
initializes the client.

For now, manually set it, which is a bit hacky because it
requires some casting & manually invoking an option function
that's technically meant for initialization. In practice, this
is pretty safe - the cast is checked defensively and we ignore
any errors (which shouldn't be possible anyway).

Signed-off-by: Milas Bowman <milas.bowman@docker.com>

cmd/main.go

@@ -24,6 +24,7 @@ import (
 	"github.com/docker/cli/cli-plugins/plugin"
 	"github.com/docker/cli/cli/command"
 	"github.com/docker/compose/v2/cmd/cmdtrace"
+	"github.com/docker/docker/client"
 	"github.com/spf13/cobra"
 
 	"github.com/docker/compose/v2/cmd/compatibility"
@@ -42,6 +43,9 @@ func pluginMain() {
 			if err := plugin.PersistentPreRunE(cmd, args); err != nil {
 				return err
 			}
+			// compose-specific initialization
+			dockerCliPostInitialize(dockerCli)
+
 			// TODO(milas): add an env var to enable logging from the
 			// OTel components for debugging purposes
 			_ = cmdtrace.Setup(cmd, dockerCli, os.Args[1:])
@@ -67,6 +71,22 @@ 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:])...)