Add option to fetch Docker-Content-Digest

Code reused from manifest.go; this one is helpful to figure out if tags
have changed

README.md

@@ -46,6 +46,7 @@ AUTHOR:
 
 COMMANDS:
      delete, rm       delete a specific reference of a repository
+     digest           get the Docker-Content-Digest
      layer, download  download a layer for the specific reference of a repository
      list, ls         list all repositories
      manifest         get the json manifest for the specific reference of a repository
@@ -102,6 +103,12 @@ latest
 stable
 ```
 
+## Get the Docker-Content-Digest
+```console
+$ reg digest htop
+"sha256:791158756cc0f5b27ef8c5c546284568fc9b7f4cf1429fb736aff3ee2d2e340f"
+```
+
 ## Get a Manifest
 
 ```console

digest.go

@@ -0,0 +1,39 @@
+package main
+
+import (
+	"encoding/json"
+	"fmt"
+
+	"github.com/genuinetools/reg/repoutils"
+	"github.com/urfave/cli"
+)
+
+var digestCommand = cli.Command{
+	Name:  "digest",
+	Usage: "get the Docker-Content-Digest",
+	Action: func(c *cli.Context) error {
+		if len(c.Args()) < 1 {
+			return fmt.Errorf("pass the name of the repository")
+		}
+
+		repo, ref, err := repoutils.GetRepoAndRef(c.Args()[0])
+		if err != nil {
+			return err
+		}
+
+		var digest interface{}
+		digest, err = r.Digest(repo, ref)
+		if err != nil {
+			return err
+		}
+
+		b, err := json.MarshalIndent(digest, " ", "  ")
+		if err != nil {
+			return err
+		}
+
+		fmt.Println(string(b))
+
+		return nil
+	},
+}

main.go

@@ -57,6 +57,7 @@ func main() {
 
 	app.Commands = []cli.Command{
 		deleteCommand,
+		digestCommand,
 		layerCommand,
 		listCommand,
 		manifestCommand,