Move ContainerFlags to "commands" package

Commit 05513284e7 exposed the "rootfs"
and "no-pivot" flags for the "containers" command, but it accidentally
removed them for "run" since package-level variables are initialized
before package-level init functions in golang. Hoisting these flags to
a package imported by both commands solves the problem.

Signed-off-by: Felix Abecassis <fabecassis@nvidia.com>

cmd/ctr/commands/containers/containers.go

@@ -49,7 +49,7 @@ var createCommand = cli.Command{
 	Name:      "create",
 	Usage:     "create container",
 	ArgsUsage: "[flags] Image|RootFS CONTAINER",
-	Flags:     append(commands.SnapshotterFlags, run.ContainerFlags...),
+	Flags:     append(commands.SnapshotterFlags, commands.ContainerFlags...),
 	Action: func(context *cli.Context) error {
 		var (
 			id  = context.Args().Get(1)