Specify Ruby image in FailFast template

Image can still be overridden, but specifying ruby here ensures that
the job succeeds by default

changelogs/unreleased/specify-ruby-image-in-fail-fast-template.yml

+---
+title: Specify Ruby image in FailFast template
+merge_request: 38523
+author:
+type: changed

lib/gitlab/ci/templates/Verify/FailFast.gitlab-ci.yml

 rspec-rails-modified-path-specs:
+  image: ruby:2.6
   stage: .pre
   rules:
     - if: $CI_MERGE_REQUEST_EVENT_TYPE == "merged_result" || $CI_MERGE_REQUEST_EVENT_TYPE == "merge_train"