LCOW: Capture stderr on external process. Log actual error rather than throwaway

Signed-off-by: John Howard <jhoward@microsoft.com>

builder/remotecontext/detect.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"io"
 	"os"
+	"runtime"
 	"strings"
 
 	"github.com/containerd/continuity/driver"
@@ -173,6 +174,9 @@ func StatAt(remote builder.Source, path string) (os.FileInfo, error) {
 func FullPath(remote builder.Source, path string) (string, error) {
 	fullPath, err := remote.Root().ResolveScopedPath(path, true)
 	if err != nil {
+		if runtime.GOOS == "windows" {
+			return "", fmt.Errorf("failed to resolve scoped path %s (%s): %s. Possible cause is a forbidden path outside the build context", path, fullPath, err)
+		}
 		return "", fmt.Errorf("Forbidden path outside the build context: %s (%s)", path, fullPath) // backwards compat with old error
 	}
 	return fullPath, nil

daemon/graphdriver/lcow/lcow_svm.go

@@ -3,6 +3,7 @@
 package lcow // import "github.com/docker/docker/daemon/graphdriver/lcow"
 
 import (
+	"bytes"
 	"errors"
 	"fmt"
 	"io"
@@ -385,7 +386,15 @@ func (svm *serviceVM) deleteUnionMount(mountName string, disks ...hcsshim.Mapped
 }
 
 func (svm *serviceVM) runProcess(command string, stdin io.Reader, stdout io.Writer, stderr io.Writer) error {
-	process, err := svm.config.RunProcess(command, stdin, stdout, stderr)
+	var process hcsshim.Process
+	var err error
+	errOut := &bytes.Buffer{}
+
+	if stderr != nil {
+		process, err = svm.config.RunProcess(command, stdin, stdout, stderr)
+	} else {
+		process, err = svm.config.RunProcess(command, stdin, stdout, errOut)
+	}
 	if err != nil {
 		return err
 	}
@@ -398,7 +407,12 @@ func (svm *serviceVM) runProcess(command string, stdin io.Reader, stdout io.Writ
 	}
 
 	if exitCode != 0 {
-		return fmt.Errorf("svm.runProcess: command %s failed with exit code %d", command, exitCode)
+		// If the caller isn't explicitly capturing stderr output, then capture it here instead.
+		e := fmt.Sprintf("svm.runProcess: command %s failed with exit code %d", command, exitCode)
+		if stderr == nil {
+			e = fmt.Sprintf("%s. (%s)", e, errOut.String())
+		}
+		return fmt.Errorf(e)
 	}
 	return nil
 }