diff --git a/Dockerfile b/Dockerfile
index 701fc09bed..9c14792f9c 100644
--- a/Dockerfile
+++ b/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
diff --git a/Library/Homebrew/style.rb b/Library/Homebrew/style.rb
index e38b194118..d2e9dc2b63 100644
--- a/Library/Homebrew/style.rb
+++ b/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" },