Add Alpine 3.8 variants of Ruby (where possible)

See https://github.com/docker-library/ruby/pull/232

library/ruby

@@ -1,4 +1,4 @@
-# this file is generated via https://github.com/docker-library/ruby/blob/b9e495c8e400363e6cc2d4db6d944235b7989584/generate-stackbrew-library.sh
+# this file is generated via https://github.com/docker-library/ruby/blob/bb5c7101e13cfa3632dfc21245b172c205d90353/generate-stackbrew-library.sh
 
 Maintainers: Tianon Gravi <[email protected]> (@tianon),
              Joseph Ferguson <[email protected]> (@yosifkit)
@@ -14,7 +14,12 @@ Architectures: amd64, arm32v5, arm32v7, arm64v8, i386, ppc64le, s390x
 GitCommit: 8e4d795f70df296f79b1272b665cc0c970e58fe2
 Directory: 2.6-rc/stretch/slim
 
-Tags: 2.6.0-preview2-alpine3.7, 2.6-rc-alpine3.7, rc-alpine3.7, 2.6.0-preview2-alpine, 2.6-rc-alpine, rc-alpine
+Tags: 2.6.0-preview2-alpine3.8, 2.6-rc-alpine3.8, rc-alpine3.8, 2.6.0-preview2-alpine, 2.6-rc-alpine, rc-alpine
+Architectures: amd64, arm32v6, arm64v8, i386, ppc64le, s390x
+GitCommit: e7d27d514f15f3bffdc7fac383a8b83494317d47
+Directory: 2.6-rc/alpine3.8
+
+Tags: 2.6.0-preview2-alpine3.7, 2.6-rc-alpine3.7, rc-alpine3.7
 Architectures: amd64, arm32v6, arm64v8, i386, ppc64le, s390x
 GitCommit: 96967d8295f1b26a784e5893f41b5f5ccd8318eb
 Directory: 2.6-rc/alpine3.7
@@ -84,7 +89,12 @@ Architectures: amd64, arm32v5, arm32v7, i386
 GitCommit: 4fa4c114e81bf8bdb653262c033da1d3fafa9141
 Directory: 2.3/jessie/slim
 
-Tags: 2.3.7-alpine3.7, 2.3-alpine3.7, 2.3.7-alpine, 2.3-alpine
+Tags: 2.3.7-alpine3.8, 2.3-alpine3.8, 2.3.7-alpine, 2.3-alpine
+Architectures: amd64, arm32v6, arm64v8, i386, ppc64le, s390x
+GitCommit: e7d27d514f15f3bffdc7fac383a8b83494317d47
+Directory: 2.3/alpine3.8
+
+Tags: 2.3.7-alpine3.7, 2.3-alpine3.7
 Architectures: amd64, arm32v6, arm64v8, i386, ppc64le, s390x
 GitCommit: 96967d8295f1b26a784e5893f41b5f5ccd8318eb
 Directory: 2.3/alpine3.7