ctr: support --user for run/create

Signed-off-by: Ye Sijun <junnplus@gmail.com>

cmd/ctr/commands/run/run_unix.go

@@ -196,6 +196,9 @@ func NewContainer(ctx gocontext.Context, client *containerd.Client, context *cli
 		if cwd := context.String("cwd"); cwd != "" {
 			opts = append(opts, oci.WithProcessCwd(cwd))
 		}
+		if user := context.String("user"); user != "" {
+			opts = append(opts, oci.WithUser(user), oci.WithAdditionalGIDs(user))
+		}
 		if context.Bool("tty") {
 			opts = append(opts, oci.WithTTY)
 		}