composefs: return mkcomposefs stderr as part of error

Writing the error to stderr is not helpful when using the podman API.
The error would only be visable on the server which is bad and likely is
not noticed at all.

Also for the regular podman cli it would be hard for a user to know if
the stderr from mkcompose is related to the returned podman error.

To fix this capture the stderr and append it to the error as string,
this should be fine assuming mkcomposefs doesn't print a ton on stderr
which I assume is not the case.

Signed-off-by: Paul Holzinger <pholzing@redhat.com>

drivers/overlay/composefs.go

@@ -4,12 +4,14 @@
 package overlay
 
 import (
+	"bytes"
 	"encoding/binary"
 	"errors"
 	"fmt"
 	"os"
 	"os/exec"
 	"path/filepath"
+	"strings"
 	"sync"
 
 	"github.com/containers/storage/pkg/chunked/dump"
@@ -70,12 +72,18 @@ func generateComposeFsBlob(verityDigests map[string]string, toc interface{}, com
 		// a scope to close outFd before setting fsverity on the read-only fd.
 		defer outFd.Close()
 
+		errBuf := &bytes.Buffer{}
 		cmd := exec.Command(writerJson, "--from-file", "-", "/proc/self/fd/3")
 		cmd.ExtraFiles = []*os.File{outFd}
-		cmd.Stderr = os.Stderr
+		cmd.Stderr = errBuf
 		cmd.Stdin = dumpReader
 		if err := cmd.Run(); err != nil {
-			return fmt.Errorf("failed to convert json to erofs: %w", err)
+			rErr := fmt.Errorf("failed to convert json to erofs: %w", err)
+			exitErr := &exec.ExitError{}
+			if errors.As(err, &exitErr) {
+				return fmt.Errorf("%w: %s", rErr, strings.TrimSpace(errBuf.String()))
+			}
+			return rErr
 		}
 		return nil
 	}()