Change library used to encode manifest (appc#245).

Manifest was encoded using official encoding/json which does not canonical encode data.
Use library from github.com/docker/go/canonical/json instead.

glide.yaml

@@ -25,3 +25,7 @@ import:
   - specs-go
 - package: github.com/opencontainers/go-digest
   version: 21dfd564fd89c944783d00d069f33e3e7123c448
+- package: github.com/docker/go
+  version: d30aec9fd63c35133f8f79c3412ad91a3b08be06
+  subpackages:
+  - canonical/json

lib/internal/internal.go

@@ -42,6 +42,7 @@ import (
 	"github.com/appc/spec/schema"
 	appctypes "github.com/appc/spec/schema/types"
 	gzip "github.com/klauspost/pgzip"
+	canJson "github.com/docker/go/canonical/json"
 )
 
 // Docker2ACIBackend is the interface that abstracts converting Docker layers
@@ -720,7 +721,7 @@ func subtractWhiteouts(pathWhitelist []string, whiteouts []string) []string {
 
 // WriteManifest writes a schema.ImageManifest entry on a tar.Writer.
 func WriteManifest(outputWriter *tar.Writer, manifest schema.ImageManifest) error {
-	b, err := json.Marshal(manifest)
+	b, err := canJson.Marshal(manifest)
 	if err != nil {
 		return err
 	}