Update ruby
Update ruby

Changes:

- https://github.com/docker-library/ruby/commit/644570a: Merge pull request https://github.com/docker-library/ruby/pull/321 from J0WI/alpine-3.12
- https://github.com/docker-library/ruby/commit/27938af: Add Alpine 3.12
- https://github.com/docker-library/ruby/commit/a6b23d5: Merge pull request https://github.com/docker-library/ruby/pull/317 from docker-library/github-actions
- https://github.com/docker-library/ruby/commit/3bda762: Add initial GitHub Actions CI

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