ci: explicit error handling on remaining errcheck call sites

The previous .golangci.yml exclude-functions list only matches
interface-typed call sites (e.g. (io.Closer).Close); concrete-type
calls like *tar.Writer.Close, *gzip.Reader.Close, and
http.ResponseWriter.Write still triggered errcheck.

Rather than enumerate every concrete type, fix the 9 remaining call
sites surgically:

  examples/*/main.go             defer rt.Close() →
                                 defer func() { _ = rt.Close() }()
                                 defer eng.Down(...) → log on error
                                 (down is a real call worth surfacing
                                 in examples).
  feature/extract.go             defer gz.Close() (gzip.Reader)
  runtime/docker/build.go        defer tw.Close() (tar.Writer)
  feature/https_test.go          w.Write in httptest handler

All wrapped with `_ =` since the errors are not actionable in their
contexts. Tests still pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: bilby91

examples/compose/main.go

@@ -67,7 +67,7 @@ services:
 	if err != nil {
 		log.Fatalf("docker daemon: %v", err)
 	}
-	defer rt.Close()
+	defer func() { _ = rt.Close() }()
 
 	eng, err := devcontainer.New(devcontainer.EngineOptions{Runtime: rt})
 	if err != nil {
@@ -82,10 +82,14 @@ services:
 	if err != nil {
 		log.Fatalf("up: %v", err)
 	}
-	defer eng.Down(context.Background(), ws, devcontainer.DownOptions{
-		Remove:        true,
-		RemoveVolumes: true,
-	})
+	defer func() {
+		if err := eng.Down(context.Background(), ws, devcontainer.DownOptions{
+			Remove:        true,
+			RemoveVolumes: true,
+		}); err != nil {
+			log.Printf("down: %v", err)
+		}
+	}()
 
 	fmt.Printf("workspace: %s\nprimary container: %s\n", ws.ID, ws.Container.ID)
 	fmt.Printf("compose project: %s\n", ws.Container.Labels["com.docker.compose.project"])

examples/image-source/main.go

@@ -47,7 +47,7 @@ func main() {
 	if err != nil {
 		log.Fatalf("docker daemon: %v", err)
 	}
-	defer rt.Close()
+	defer func() { _ = rt.Close() }()
 
 	eng, err := devcontainer.New(devcontainer.EngineOptions{Runtime: rt})
 	if err != nil {
@@ -63,7 +63,11 @@ func main() {
 	if err != nil {
 		log.Fatalf("up: %v", err)
 	}
-	defer eng.Down(context.Background(), ws, devcontainer.DownOptions{Remove: true})
+	defer func() {
+		if err := eng.Down(context.Background(), ws, devcontainer.DownOptions{Remove: true}); err != nil {
+			log.Printf("down: %v", err)
+		}
+	}()
 
 	fmt.Printf("workspace id: %s\ncontainer id: %s\n", ws.ID, ws.Container.ID)
 

examples/with-features/main.go

@@ -68,7 +68,7 @@ echo example-feature-installed > /etc/example-feature-marker
 	if err != nil {
 		log.Fatalf("docker daemon: %v", err)
 	}
-	defer rt.Close()
+	defer func() { _ = rt.Close() }()
 
 	eng, err := devcontainer.New(devcontainer.EngineOptions{Runtime: rt})
 	if err != nil {
@@ -83,7 +83,11 @@ echo example-feature-installed > /etc/example-feature-marker
 	if err != nil {
 		log.Fatalf("up: %v", err)
 	}
-	defer eng.Down(context.Background(), ws, devcontainer.DownOptions{Remove: true})
+	defer func() {
+		if err := eng.Down(context.Background(), ws, devcontainer.DownOptions{Remove: true}); err != nil {
+			log.Printf("down: %v", err)
+		}
+	}()
 
 	fmt.Printf("workspace: %s\ncontainer: %s\n", ws.ID, ws.Container.ID)
 

feature/extract.go

@@ -25,7 +25,7 @@ func extractTarball(r io.Reader, dst string) error {
 	if err != nil {
 		return fmt.Errorf("gzip: %w", err)
 	}
-	defer gz.Close()
+	defer func() { _ = gz.Close() }()
 	return extractTar(gz, dst)
 }
 

feature/https_test.go

@@ -24,7 +24,7 @@ func TestFetchHTTPS_HappyPath(t *testing.T) {
 			http.NotFound(w, r)
 			return
 		}
-		w.Write(body)
+		_, _ = w.Write(body)
 	}))
 	defer srv.Close()
 

runtime/docker/build.go

@@ -137,7 +137,7 @@ func streamBuildOutput(ctx context.Context, body io.ReadCloser, events chan<- ru
 // error rather than silently dropped.
 func tarDirectory(dir string, w io.Writer) error {
 	tw := tar.NewWriter(w)
-	defer tw.Close()
+	defer func() { _ = tw.Close() }()
 
 	return filepath.WalkDir(dir, func(path string, d fs.DirEntry, err error) error {
 		if err != nil {