Add a "metadata" file contents flag

Currently this supports "yaml" as the only option, which will output
the yaml config (as JSON) into the file specified in the image.

Fix #107

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

src/moby/build.go

@@ -3,6 +3,7 @@ package moby
 import (
 	"archive/tar"
 	"bytes"
+	"encoding/json"
 	"errors"
 	"fmt"
 	"io"
@@ -359,6 +360,16 @@ func tarAppend(iw *tar.Writer, tr *tar.Reader) error {
 	return nil
 }
 
+// this allows inserting metadata into a file in the image
+func metadata(m Moby, md string) ([]byte, error) {
+	switch md {
+	case "yaml":
+		return json.MarshalIndent(m, "", "    ")
+	default:
+		return []byte{}, fmt.Errorf("Unsupported metadata type: %s", md)
+	}
+}
+
 func filesystem(m Moby, tw *tar.Writer, idMap map[string]uint32) error {
 	// TODO also include the files added in other parts of the build
 	var addedFiles = map[string]bool{}
@@ -410,10 +421,14 @@ func filesystem(m Moby, tw *tar.Writer, idMap map[string]uint32) error {
 		if f.Contents != nil {
 			contents = []byte(*f.Contents)
 		}
-		if !f.Directory && f.Contents == nil && f.Symlink == "" {
-			if f.Source == "" {
-				return errors.New("Contents of file not specified")
+		if !f.Directory && f.Symlink == "" && f.Contents == nil {
+			if f.Source == "" && f.Metadata == "" {
+				return fmt.Errorf("Contents of file (%s) not specified", f.Path)
 			}
+			if f.Source != "" && f.Metadata != "" {
+				return fmt.Errorf("Specified Source and Metadata for file: %s", f.Path)
+			}
+			if f.Source != "" {
 				if len(f.Source) > 2 && f.Source[:2] == "~/" {
 					f.Source = homeDir() + f.Source[1:]
 				}
@@ -430,6 +445,19 @@ func filesystem(m Moby, tw *tar.Writer, idMap map[string]uint32) error {
 				if err != nil {
 					return err
 				}
+			} else {
+				contents, err = metadata(m, f.Metadata)
+				if err != nil {
+					return err
+				}
+			}
+		} else {
+			if f.Metadata != "" {
+				return fmt.Errorf("Specified Contents and Metadata for file: %s", f.Path)
+			}
+			if f.Source != "" {
+				return fmt.Errorf("Specified Contents and Source for file: %s", f.Path)
+			}
 		}
 		// we need all the leading directories
 		parts := strings.Split(path.Dir(f.Path), "/")

src/moby/config.go

@@ -48,6 +48,7 @@ type File struct {
 	Symlink   string
 	Contents  *string
 	Source    string
+	Metadata  string
 	Optional  bool
 	Mode      string
 	UID       interface{} `yaml:"uid" json:"uid"`

src/moby/schema.go

@@ -25,6 +25,7 @@ var schema = string(`
           "symlink": {"type": "string"},
           "contents": {"type": "string"},
           "source": {"type": "string"},
+          "metadata": {"type": "string"},
           "optional": {"type": "boolean"},
           "mode": {"type": "string"},
           "uid": {"anyOf": [{"type": "string"}, {"type": "integer"}]},