chore(deps): update postgres:18.6-alpine docker digest to 6c538e7 (#308)

k8s/infra/postgres.yaml

@@ -53,7 +53,7 @@ spec:
     spec:
       containers:
       - name: postgres
-        image: postgres:18.6-alpine@sha256:650d7a867dc5336eff67802a1417fcc801821d73d824c608e80afbde792fe6e1
+        image: postgres:18.6-alpine@sha256:6c538e7206ea40ff740ef27883529390a690b6ead6ba96b44c67a9f7c638e8fd
         args:
         - -c
         - shared_buffers=384MB