Meta: In HN, prefix a line with 2 spaces to get code formatting, ex.

  # syntax=docker/dockerfile:1.3-labs
  FROM alpine
  RUN <<EOF
  echo "This is a heredoc example"
  echo "It allows multi-line commands"
  EOF

Non-meta: Do you happen to know how portable that is across old docker, podman/buildah, kaniko, etc.? I'd like to adopt it but I don't want it to bite me when I'm not running a recent version of literal docker.

It is new feature and not portable to old versions. But modern podman supports it. No idea about kaniko.