Bump docker-ruby from 15cf9d7 to 559bc12 (#3)

Bumps [docker-ruby](https://github.com/docker-library/ruby) from `15cf9d7` to `559bc12`.
- [Commits](docker-library/ruby@15cf9d7...559bc12)

---
updated-dependencies:
- dependency-name: docker-ruby
  dependency-type: direct:production
...

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