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/snapshots/snapshots.go

@@ -280,6 +280,10 @@ var prepareCommand = cli.Command{
 			Name:  "target, t",
 			Usage: "mount target path, will print mount, if provided",
 		},
+		cli.BoolFlag{
+			Name:  "mounts",
+			Usage: "Print out snapshot mounts as JSON",
+		},
 	},
 	Action: func(context *cli.Context) error {
 		if narg := context.NArg(); narg < 1 || narg > 2 {
@@ -310,6 +314,10 @@ var prepareCommand = cli.Command{
 			printMounts(target, mounts)
 		}
 
+		if context.Bool("mounts") {
+			commands.PrintAsJSON(mounts)
+		}
+
 		return nil
 	},
 }
@@ -323,6 +331,10 @@ var viewCommand = cli.Command{
 			Name:  "target, t",
 			Usage: "mount target path, will print mount, if provided",
 		},
+		cli.BoolFlag{
+			Name:  "mounts",
+			Usage: "Print out snapshot mounts as JSON",
+		},
 	},
 	Action: func(context *cli.Context) error {
 		if narg := context.NArg(); narg < 1 || narg > 2 {
@@ -349,6 +361,10 @@ var viewCommand = cli.Command{
 			printMounts(target, mounts)
 		}
 
+		if context.Bool("mounts") {
+			commands.PrintAsJSON(mounts)
+		}
+
 		return nil
 	},
 }