Bump dockerfile-rails from 1.7.6 to 1.7.9 (#197)

Bumps [dockerfile-rails](https://github.com/fly-apps/dockerfile-rails) from 1.7.6 to 1.7.9.
- [Release notes](https://github.com/fly-apps/dockerfile-rails/releases)
- [Commits](fly-apps/dockerfile-rails@v1.7.6...v1.7.9)

---
updated-dependencies:
- dependency-name: dockerfile-rails
  dependency-version: 1.7.9
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

Gemfile.lock

@@ -122,7 +122,7 @@ GEM
     debug (1.10.0)
       irb (~> 1.10)
       reline (>= 0.3.8)
-    dockerfile-rails (1.7.6)
+    dockerfile-rails (1.7.9)
       rails (>= 3.0.0)
     drb (2.2.1)
     erubi (1.13.1)