Decouple (skopeo inspect) output formatting from types.Image.Manifest

Does not change behavior.

This will allow us to move collecting some of the data to the (skopeo
inspect) code and to have a more focused types.Image API, where
types.Image.Manifest() does not return a grab bag of manifest-unrelated
data, eventually.

For how it actually makes the coupling more explicit by having
types.Image.Manifest() return a types.DockerImageManifest instead of the
too generic types.ImageManifest.  We will need to think about which
parts of DockerImageManifest are truly generic, later.

cmd/skopeo/inspect.go

@@ -3,11 +3,26 @@ package main
 import (
 	"encoding/json"
 	"fmt"
+	"time"
 
 	"github.com/Sirupsen/logrus"
 	"github.com/codegangsta/cli"
 )
 
+// inspectOutput is the output format of (skopeo inspect), primarily so that we can format it with a simple json.MarshalIndent.
+type inspectOutput struct {
+	Name          string
+	Tag           string
+	Digest        string
+	RepoTags      []string
+	Created       time.Time
+	DockerVersion string
+	Labels        map[string]string
+	Architecture  string
+	Os            string
+	Layers        []string
+}
+
 var inspectCmd = cli.Command{
 	Name:  "inspect",
 	Usage: "inspect images on a registry",
@@ -34,7 +49,19 @@ var inspectCmd = cli.Command{
 		if err != nil {
 			logrus.Fatal(err)
 		}
-		out, err := json.MarshalIndent(imgInspect, "", "    ")
+		outputData := inspectOutput{
+			Name:          imgInspect.Name,
+			Tag:           imgInspect.Tag,
+			Digest:        imgInspect.Digest,
+			RepoTags:      imgInspect.RepoTags,
+			Created:       imgInspect.Created,
+			DockerVersion: imgInspect.DockerVersion,
+			Labels:        imgInspect.Labels,
+			Architecture:  imgInspect.Architecture,
+			Os:            imgInspect.Os,
+			Layers:        imgInspect.Layers,
+		}
+		out, err := json.MarshalIndent(outputData, "", "    ")
 		if err != nil {
 			logrus.Fatal(err)
 		}

docker/docker_image.go

@@ -62,7 +62,7 @@ func (i *dockerImage) Signatures() ([][]byte, error) {
 	return i.cachedSignatures, nil
 }
 
-func (i *dockerImage) Inspect() (types.ImageManifest, error) {
+func (i *dockerImage) Inspect() (*types.DockerImageManifest, error) {
 	// TODO(runcom): unused version param for now, default to docker v2-1
 	m, err := i.getSchema1Manifest()
 	if err != nil {
@@ -122,7 +122,7 @@ type v1Image struct {
 	OS string `json:"os,omitempty"`
 }
 
-func makeImageManifest(name string, m *manifestSchema1, dgst string, tagList []string) (types.ImageManifest, error) {
+func makeImageManifest(name string, m *manifestSchema1, dgst string, tagList []string) (*types.DockerImageManifest, error) {
 	v1 := &v1Image{}
 	if err := json.Unmarshal([]byte(m.History[0].V1Compatibility), v1); err != nil {
 		return nil, err

types/types.go

@@ -62,16 +62,10 @@ type Image interface {
 	// Signatures is like ImageSource.GetSignatures, but the result is cached; it is OK to call this however often you need.
 	Signatures() ([][]byte, error)
 	Layers(layers ...string) error // configure download directory? Call it DownloadLayers?
-	Inspect() (ImageManifest, error)
+	Inspect() (*DockerImageManifest, error)
 	DockerTar() ([]byte, error) // ??? also, configure output directory
 }
 
-// ImageManifest is the interesting subset of metadata about an Image.
-// TODO(runcom)
-type ImageManifest interface {
-	String() string
-}
-
 // DockerImageManifest is a set of metadata describing Docker images and their manifest.json files.
 // Note that this is not exactly manifest.json, e.g. some fields have been added.
 type DockerImageManifest struct {