Escaped dollar signs in heredoc blocks are not recognized correctly.

[lucassus]

• This is a bug report
• This is a feature request
• I searched existing issues before opening this one

I have the following RUN with heredoc in my Dockerfile

ARG WKHTMLTOPDF_VERSION="0.12.6.1-3"

# Install patched wkhtmltopdf for PDF generation, see https://github.com/wkhtmltopdf/packaging/releases
RUN <<EOF bash
  set -eux

  # We need to escape dollar sign in variables to prevent it from being expanded during the build.
  # For instance use \${version_codename}.
  version_codename=$(grep --only-matching --perl-regexp '(?<=VERSION_CODENAME=).+$' /etc/os-release)
  download_url="https://github.com/wkhtmltopdf/packaging/releases/download/${WKHTMLTOPDF_VERSION}/wkhtmltox_${WKHTMLTOPDF_VERSION}.\${version_codename}_${BUILDARCH}.deb"

  curl \${download_url} --location --output /tmp/wkhtmltopdf.deb
  apt-get install --yes --no-install-recommends /tmp/wkhtmltopdf.deb
  rm /tmp/wkhtmltopdf.deb
EOF

It works fine. But the linter complains with:

-:48 SC1083 warning: This } is literal. Check expression (missing ;/\n?) or quote it.

When I quote download_url variable, like:

curl "\${download_url}" --location --output /tmp/wkhtmltopdf.deb

I have another warning:

-:48 SC2034 warning: download_url appears unused. Verify use (or export if used externally).

Expected behavior

The linter should understand that I have to escape a dollar sign in this case.