Update ruby
Update ruby

Changes:

- https://github.com/docker-library/ruby/commit/d6c1778: Update to 2.4.8, rubygems 3.0.3
- https://github.com/docker-library/ruby/commit/21c98da: Update to 2.5.7, rubygems 3.0.3
- https://github.com/docker-library/ruby/commit/8565a59: Update to 2.6.5
- https://github.com/docker-library/ruby/commit/1382674: Merge pull request https://github.com/docker-library/ruby/pull/296 from JanDintel/ruby-version-upgrades
- https://github.com/docker-library/ruby/commit/5a3b107: Update ruby 2.4.x to 2.4.8
- https://github.com/docker-library/ruby/commit/5c9e21c: Update ruby 2.6.x to 2.6.5
- https://github.com/docker-library/ruby/commit/bf0e16e: Update ruby 2.5.x to 2.5.7
- https://github.com/docker-library/ruby/commit/704b155: Update generated README

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