False negatives for SC2086 - Double quote to prevent globbing and word splitting

[ento]

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

Expected behavior

Running hadolint against this Dockerfile should report SC2086

FROM debian:10.0
ARG FOO
RUN python foo.py --foo $FOO

Actual behavior

SC2086 doesn't get reported

Steps to reproduce the behavior

Output of hadolint --version :

Installed via nix-shell

$ hadolint --version
Haskell Dockerfile Linter 2.12.0

Dockerfile (if relevant)

FROM debian:10.0
ARG FOO
RUN python foo.py --foo $FOO

Additional environment details (OS, stack version, etc.)

shellcheck's online playground reports SC2086 when you enter a similar script

#!/bin/sh
FOO=
python foo.py --foo $FOO

Removing ARG makes hadolint report SC2086

FROM debian:10.0 as debian
RUN python foo.py --foo $FOO

$ hadolint Dockerfile
Dockerfile:2 SC2086 info: Double quote to prevent globbing and word splitting.