Update ghost

Changes:

- https://github.com/docker-library/ghost/commit/79c91f0: Update to 4.21.0, ghost-cli 1.18.0
- https://github.com/docker-library/ghost/commit/ef76a61: Update to 3.42.6, ghost-cli 1.18.0
- https://github.com/docker-library/ghost/commit/bafe441: Update to 4.20.4, ghost-cli 1.18.0

library/ghost

@@ -4,22 +4,22 @@ Maintainers: Tianon Gravi <[email protected]> (@tianon),
              Joseph Ferguson <[email protected]> (@yosifkit)
 GitRepo: https://github.com/docker-library/ghost.git
 
-Tags: 4.20.4, 4.20, 4, latest
+Tags: 4.21.0, 4.21, 4, latest
 Architectures: amd64, arm32v7, arm64v8, ppc64le, s390x
-GitCommit: b0a55c8c18a1f81ef8714caf56b5120e6d362ed0
+GitCommit: 79c91f0e74511e5e7bbea3be157369e14e71e3a2
 Directory: 4/debian
 
-Tags: 4.20.4-alpine, 4.20-alpine, 4-alpine, alpine
+Tags: 4.21.0-alpine, 4.21-alpine, 4-alpine, alpine
 Architectures: amd64, arm32v6, arm32v7, arm64v8
-GitCommit: b0a55c8c18a1f81ef8714caf56b5120e6d362ed0
+GitCommit: 79c91f0e74511e5e7bbea3be157369e14e71e3a2
 Directory: 4/alpine
 
 Tags: 3.42.6, 3.42, 3
 Architectures: amd64, arm32v7, arm64v8, ppc64le, s390x
-GitCommit: fbd1efc8ea0a9872e80c3746b9724e522a96b584
+GitCommit: ef76a614ab4ba18094998987a76935790af2697b
 Directory: 3/debian
 
 Tags: 3.42.6-alpine, 3.42-alpine, 3-alpine
 Architectures: amd64, arm32v6, arm32v7, arm64v8
-GitCommit: fbd1efc8ea0a9872e80c3746b9724e522a96b584
+GitCommit: ef76a614ab4ba18094998987a76935790af2697b
 Directory: 3/alpine