Support creating of directories in files section

e.g.
    files:
     - path: etc/docker/daemon.json
       contents: '{"debug": true}'
     - path: var/lib/mydaemon
       directory: true

Signed-off-by: Ian Campbell <ian.campbell@docker.com>

src/cmd/moby/config.go

@@ -29,6 +29,7 @@ type Moby struct {
 	Daemon []MobyImage
 	Files  []struct {
 		Path      string
+		Directory bool
 		Contents  string
 	}
 	Outputs []struct {
@@ -385,7 +386,7 @@ func filesystem(m *Moby) (*bytes.Buffer, error) {
 		if f.Path == "" {
 			return buf, errors.New("Did not specify path for file")
 		}
-		if f.Contents == "" {
+		if !f.Directory && f.Contents == "" {
 			return buf, errors.New("Contents of file not specified")
 		}
 		// we need all the leading directories
@@ -410,6 +411,21 @@ func filesystem(m *Moby) (*bytes.Buffer, error) {
 				return buf, err
 			}
 		}
+
+		if f.Directory {
+			if f.Contents != "" {
+				return buf, errors.New("Directory with contents not allowed")
+			}
+			hdr := &tar.Header{
+				Name:     f.Path,
+				Typeflag: tar.TypeDir,
+				Mode:     0700,
+			}
+			err := tw.WriteHeader(hdr)
+			if err != nil {
+				return buf, err
+			}
+		} else {
 			hdr := &tar.Header{
 				Name: f.Path,
 				Mode: 0600,
@@ -424,5 +440,6 @@ func filesystem(m *Moby) (*bytes.Buffer, error) {
 				return buf, err
 			}
 		}
+	}
 	return buf, nil
 }