Push docker manifest for discourse/base:2.0.${{ env.TIMESTAMP }}-15 (#926)

Follow-up to ab957b2

Author: tgxworld

.github/workflows/build.yml

@@ -202,6 +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 }}-stable
           docker manifest push discourse/base:release
           docker manifest push discourse/base:release-stable