style: brew style check Dockerfile as shell script

Dockerfile

@@ -1,5 +1,6 @@
 ARG version=20.04
-FROM ubuntu:$version
+# shellcheck disable=SC2154
+FROM ubuntu:"${version}"
 ARG DEBIAN_FRONTEND=noninteractive
 
 # hadolint ignore=DL3008

Library/Homebrew/style.rb

@@ -177,6 +177,7 @@ module Homebrew
         files = [
           HOMEBREW_BREW_FILE,
           HOMEBREW_REPOSITORY/"completions/bash/brew",
+          HOMEBREW_REPOSITORY/"Dockerfile",
           *HOMEBREW_LIBRARY.glob("Homebrew/*.sh"),
           *HOMEBREW_LIBRARY.glob("Homebrew/shims/**/*").map(&:realpath).uniq
                            .reject { |path| path.directory? || path.basename.to_s == "cc" },