Update html-proofer requirement from ~> 3 to ~> 5 in /.docker/entrypoint

Updates the requirements on [html-proofer](https://github.com/gjtorikian/html-proofer) to permit the latest version.
- [Release notes](https://github.com/gjtorikian/html-proofer/releases)
- [Changelog](https://github.com/gjtorikian/html-proofer/blob/main/CHANGELOG.md)
- [Commits](gjtorikian/html-proofer@v3.0.0...v5.0.0)

---
updated-dependencies:
- dependency-name: html-proofer
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

.docker/entrypoint/Gemfile

@@ -14,7 +14,7 @@ gem 'rouge', '~> 4'
 group :development, :test do
   gem 'codecov', '>= 0.6'
   gem 'diffy', '~> 3'
-  gem 'html-proofer', '~> 3'
+  gem 'html-proofer', '~> 5'
   gem 'html-proofer-unrendered-markdown', '>= 0.1'
   gem 'its', '>= 0.2'
   gem 'rake', '~> 13'