Bump dockerfile-rails from 1.6.8 to 1.6.9

Bumps [dockerfile-rails](https://github.com/fly-apps/dockerfile-rails) from 1.6.8 to 1.6.9.
- [Release notes](https://github.com/fly-apps/dockerfile-rails/releases)
- [Commits](fly-apps/dockerfile-rails@v1.6.8...v1.6.9)

---
updated-dependencies:
- dependency-name: dockerfile-rails
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

Gemfile.lock

@@ -136,7 +136,7 @@ GEM
     devise-i18n (1.12.0)
       devise (>= 4.9.0)
     diff-lcs (1.5.1)
-    dockerfile-rails (1.6.8)
+    dockerfile-rails (1.6.9)
       rails (>= 3.0.0)
     dry-core (0.9.1)
       concurrent-ruby (~> 1.0)