FIX: use appropriate manifest for pg-15 image from 8a956d4

Follow-up to ab957b2

Author: mwaniki-wairungu

.github/workflows/build.yml

@@ -202,7 +202,7 @@ jobs:
           docker manifest push discourse/base:2.0.${{ env.TIMESTAMP }}-slim
           docker manifest push discourse/base:slim
           docker manifest push discourse/base:2.0.${{ env.TIMESTAMP }}
-          docker manifest push discourse/base:2.0.${{ env.TIMESTAMP }}-15
+          docker manifest push discourse/base:2.0.${{ env.TIMESTAMP }}-pg-15
           docker manifest push discourse/base:2.0.${{ env.TIMESTAMP }}-stable
           docker manifest push discourse/base:release
           docker manifest push discourse/base:release-stable