Update ghost

Changes:

- https://github.com/docker-library/ghost/commit/38f1500: Update to 3.15.0, ghost-cli 1.13.1
- https://github.com/docker-library/ghost/commit/03c59c7: Merge pull request https://github.com/docker-library/ghost/pull/218 from docker-library/github-actions
- https://github.com/docker-library/ghost/commit/25ab22f: Add initial GitHub Actions CI

library/ghost

@@ -4,14 +4,14 @@ Maintainers: Tianon Gravi <[email protected]> (@tianon),
              Joseph Ferguson <[email protected]> (@yosifkit)
 GitRepo: https://github.com/docker-library/ghost.git
 
-Tags: 3.14.0, 3.14, 3, latest
+Tags: 3.15.0, 3.15, 3, latest
 Architectures: amd64, arm32v7, arm64v8, ppc64le, s390x
-GitCommit: 29e0b3ccd4b502e88e09ff1b2a67cd526ed13124
+GitCommit: 38f1500b0b0d395a91cb286b788e96b2c21be1ca
 Directory: 3/debian
 
-Tags: 3.14.0-alpine, 3.14-alpine, 3-alpine, alpine
+Tags: 3.15.0-alpine, 3.15-alpine, 3-alpine, alpine
 Architectures: amd64, arm32v6, arm32v7, arm64v8, i386, ppc64le, s390x
-GitCommit: 29e0b3ccd4b502e88e09ff1b2a67cd526ed13124
+GitCommit: 38f1500b0b0d395a91cb286b788e96b2c21be1ca
 Directory: 3/alpine
 
 Tags: 2.38.1, 2.38, 2