Update ghost

Changes:

- https://github.com/docker-library/ghost/commit/6558fd3: Update to 4.9.1, ghost-cli 1.17.3
- https://github.com/docker-library/ghost/commit/8959c45: Update to 4.9.0, ghost-cli 1.17.3
- https://github.com/docker-library/ghost/commit/e3b278a: Update README

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: 4.8.4, 4.8, 4, latest
+Tags: 4.9.1, 4.9, 4, latest
 Architectures: amd64, arm32v7, arm64v8, ppc64le, s390x
-GitCommit: 2855620d52fd2d5898a8f9064c898045f6402011
+GitCommit: 6558fd3d33246c2543b20863574e9c9ef25d049c
 Directory: 4/debian
 
-Tags: 4.8.4-alpine, 4.8-alpine, 4-alpine, alpine
+Tags: 4.9.1-alpine, 4.9-alpine, 4-alpine, alpine
 Architectures: amd64, arm32v6, arm32v7, arm64v8
-GitCommit: 2855620d52fd2d5898a8f9064c898045f6402011
+GitCommit: 6558fd3d33246c2543b20863574e9c9ef25d049c
 Directory: 4/alpine
 
 Tags: 3.42.5, 3.42, 3