Do not error if cache/config directory not specified

Just use the default. Easier to use as an external library.

Signed-off-by: Justin Cormack <justin.cormack@docker.com>

src/moby/build.go

@@ -142,7 +142,7 @@ func outputImage(image *Image, section string, prefix string, m Moby, idMap map[
 // Build performs the actual build process
 func Build(m Moby, w io.Writer, pull bool, tp string) error {
 	if MobyDir == "" {
-		return fmt.Errorf("MobyDir for temporary storage not set")
+		MobyDir = defaultMobyConfigDir()
 	}
 
 	iw := tar.NewWriter(w)

src/moby/util.go

@@ -1,6 +1,16 @@
 package moby
 
+import (
+	"path/filepath"
+)
+
 var (
-	// MobyDir is the location of the cache directory which should be set by the caller
+	// MobyDir is the location of the cache directory, defaults to ~/.moby
 	MobyDir string
 )
+
+func defaultMobyConfigDir() string {
+	mobyDefaultDir := ".moby"
+	home := homeDir()
+	return filepath.Join(home, mobyDefaultDir)
+}