Bump dockerfile-rails from 1.6.19 to 1.6.20

Bumps [dockerfile-rails](https://github.com/fly-apps/dockerfile-rails) from 1.6.19 to 1.6.20.
- [Release notes](https://github.com/fly-apps/dockerfile-rails/releases)
- [Commits](fly-apps/dockerfile-rails@v1.6.19...v1.6.20)

---
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

@@ -111,7 +111,7 @@ GEM
  reline (>= 0.3.8)
  diff-lcs (1.5.1)
  diffy (3.4.2)
- dockerfile-rails (1.6.19)
+ dockerfile-rails (1.6.20)
  rails (>= 3.0.0)
  dotenv (3.1.4)
  erubi (1.13.0)