Bump dockerfile-rails from 1.6.19 to 1.6.22

Bumps [dockerfile-rails](https://github.com/fly-apps/dockerfile-rails) from 1.6.19 to 1.6.22.
- [Release notes](https://github.com/fly-apps/dockerfile-rails/releases)
- [Commits](fly-apps/dockerfile-rails@v1.6.19...v1.6.22)

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

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

Gemfile.lock

@@ -105,7 +105,7 @@ GEM
       railties (>= 4.1.0)
       responders
       warden (~> 1.2.3)
-    dockerfile-rails (1.6.19)
+    dockerfile-rails (1.6.22)
       rails (>= 3.0.0)
     drb (2.2.1)
     dry-cli (1.1.0)