Add a new flag to images pull for printing out the pulled image's chainID

Signed-off-by: Kathryn Baldauf <kabaldau@microsoft.com>

cmd/ctr/commands/images/pull.go

@@ -25,6 +25,7 @@ import (
 	"github.com/containerd/containerd/images"
 	"github.com/containerd/containerd/log"
 	"github.com/containerd/containerd/platforms"
+	"github.com/opencontainers/image-spec/identity"
 	ocispec "github.com/opencontainers/image-spec/specs-go/v1"
 	"github.com/pkg/errors"
 	"github.com/urfave/cli"
@@ -57,6 +58,10 @@ command. As part of this process, we do the following:
 			Name:  "all-metadata",
 			Usage: "Pull metadata for all platforms",
 		},
+		cli.BoolFlag{
+			Name:  "print-chainid",
+			Usage: "Print the resulting image's chain ID",
+		},
 	),
 	Action: func(context *cli.Context) error {
 		var (
@@ -118,6 +123,14 @@ command. As part of this process, we do the following:
 			if err != nil {
 				return err
 			}
+			if context.Bool("print-chainid") {
+				diffIDs, err := i.RootFS(ctx)
+				if err != nil {
+					return err
+				}
+				chainID := identity.ChainID(diffIDs).String()
+				fmt.Printf("image chain ID: %s\n", chainID)
+			}
 		}
 
 		fmt.Println("done")