Update ruby

Changes:

- https://github.com/docker-library/ruby/commit/becf07f: Merge pull request https://github.com/docker-library/ruby/pull/280 from infosiftr/ruby-2.7
- https://github.com/docker-library/ruby/commit/b0c1b3c: Add 2.7.0-preview1

library/ruby

@@ -4,6 +4,21 @@ Maintainers: Tianon Gravi <[email protected]> (@tianon),
              Joseph Ferguson <[email protected]> (@yosifkit)
 GitRepo: https://github.com/docker-library/ruby.git
 
+Tags: 2.7.0-preview1-stretch, 2.7-rc-stretch, rc-stretch, 2.7.0-preview1, 2.7-rc, rc
+Architectures: amd64, arm32v5, arm32v7, arm64v8, i386, ppc64le, s390x
+GitCommit: b0c1b3c89142267912d8d0b532e206463c7d8ee0
+Directory: 2.7-rc/stretch
+
+Tags: 2.7.0-preview1-slim-stretch, 2.7-rc-slim-stretch, rc-slim-stretch, 2.7.0-preview1-slim, 2.7-rc-slim, rc-slim
+Architectures: amd64, arm32v5, arm32v7, arm64v8, i386, ppc64le, s390x
+GitCommit: b0c1b3c89142267912d8d0b532e206463c7d8ee0
+Directory: 2.7-rc/stretch/slim
+
+Tags: 2.7.0-preview1-alpine3.9, 2.7-rc-alpine3.9, rc-alpine3.9, 2.7.0-preview1-alpine, 2.7-rc-alpine, rc-alpine
+Architectures: amd64, arm32v6, arm32v7, arm64v8, i386, ppc64le, s390x
+GitCommit: b0c1b3c89142267912d8d0b532e206463c7d8ee0
+Directory: 2.7-rc/alpine3.9
+
 Tags: 2.6.3-stretch, 2.6-stretch, 2-stretch, stretch, 2.6.3, 2.6, 2, latest
 Architectures: amd64, arm32v5, arm32v7, arm64v8, i386, ppc64le, s390x
 GitCommit: 9ae0943fa2935b3a13c72ae7d6afa2439145d7fa