plugins: don't handle signal/notify if TTY

In order to solve the "double notification" issue (see:
https://github.com/docker/cli/commit/ef5e5fa03f0603f48678bfd7039c5b8121d98df1)
without running the plugin process under a new pgid (see:
https://github.com/moby/moby/issues/47073) we instead check if we're
attached to a TTY, and if so skip signalling the plugin process since it
will already be signalled.

Signed-off-by: Laura Brehm <laurabrehm@hey.com>

cmd/docker/docker.go

@@ -241,6 +241,11 @@ func tryPluginRun(dockerCli command.Cli, cmd *cobra.Command, subcommand string,
 	go func() {
 		retries := 0
 		for range signals {
+			if dockerCli.Out().IsTerminal() {
+				// running attached to a terminal, so the plugin will already
+				// receive signals due to sharing a pgid with the parent CLI
+				continue
+			}
 			if conn != nil {
 				if err := conn.Close(); err != nil {
 					_, _ = fmt.Fprintf(dockerCli.Err(), "failed to signal plugin to close: %v\n", err)