Merge pull request moby#3872 from ggjulio/update-reference-doc

Update reference.md - add set -ex to heredoc

Author: jedevc

frontend/dockerfile/docs/reference.md

@@ -2599,6 +2599,7 @@ here-doc delimiter as part of the same command.
 # syntax=docker/dockerfile:1
 FROM debian
 RUN <<EOT bash
+  set -ex
   apt-get update
   apt-get install -y vim
 EOT