driver: don't create tracer delegate opt if tracer is nil

The error handling for the cast to client.TracerDelegate was incorrect,
and previously, a client would unconditionally append an opt.

This results in the scenario that while the ClientOpt was not nil, the
tracer delegate in the ClientOpt was, which isn't an error case
explicitly handled by buildkit.

Signed-off-by: Justin Chadwell <me@jedevc.com>

driver/docker-container/driver.go

@@ -357,11 +357,14 @@ func (d *Driver) Client(ctx context.Context) (*client.Client, error) {
 		return nil, err
 	}
 
-	td, _ := exp.(client.TracerDelegate)
-
-	return client.New(ctx, "", client.WithContextDialer(func(context.Context, string) (net.Conn, error) {
+	var opts []client.ClientOpt
+	opts = append(opts, client.WithContextDialer(func(context.Context, string) (net.Conn, error) {
 		return conn, nil
-	}), client.WithTracerDelegate(td))
+	}))
+	if td, ok := exp.(client.TracerDelegate); ok {
+		opts = append(opts, client.WithTracerDelegate(td))
+	}
+	return client.New(ctx, "", opts...)
 }
 
 func (d *Driver) Factory() driver.Factory {

driver/kubernetes/driver.go

@@ -215,11 +215,14 @@ func (d *Driver) Client(ctx context.Context) (*client.Client, error) {
 		return nil, err
 	}
 
-	td, _ := exp.(client.TracerDelegate)
-
-	return client.New(ctx, "", client.WithContextDialer(func(context.Context, string) (net.Conn, error) {
+	var opts []client.ClientOpt
+	opts = append(opts, client.WithContextDialer(func(context.Context, string) (net.Conn, error) {
 		return conn, nil
-	}), client.WithTracerDelegate(td))
+	}))
+	if td, ok := exp.(client.TracerDelegate); ok {
+		opts = append(opts, client.WithTracerDelegate(td))
+	}
+	return client.New(ctx, "", opts...)
 }
 
 func (d *Driver) Factory() driver.Factory {