Update ghost

Changes:

- https://github.com/docker-library/ghost/commit/55d673e: Update to 4.15.0, ghost-cli 1.17.3

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: 4.14.0, 4.14, 4, latest
+Tags: 4.15.0, 4.15, 4, latest
 Architectures: amd64, arm32v7, arm64v8, ppc64le, s390x
-GitCommit: 33d1ce56ef2ae258b7469e0551717ad7ed9bb50a
+GitCommit: 55d673e6116840feae0cd46e97fdd06dacd590f2
 Directory: 4/debian
 
-Tags: 4.14.0-alpine, 4.14-alpine, 4-alpine, alpine
+Tags: 4.15.0-alpine, 4.15-alpine, 4-alpine, alpine
 Architectures: amd64, arm32v6, arm32v7, arm64v8
-GitCommit: 33d1ce56ef2ae258b7469e0551717ad7ed9bb50a
+GitCommit: 55d673e6116840feae0cd46e97fdd06dacd590f2
 Directory: 4/alpine
 
 Tags: 3.42.5, 3.42, 3