fix(postfix): use hash: transport_maps with postmap init container

- Splits hash-type map files into a separate postfix-maps ConfigMap
- Adds postmap init container that builds .db files from all maps into
  a postfix-db emptyDir, which is then subPath-mounted per map in the
  main container
- Updates transport_maps in main.cf to hash:/etc/postfix/transport

apps/base/mailgateway/kustomization.yaml

@@ -20,6 +20,10 @@ configMapGenerator:
     files:
       - main.cf=resources/postfix/main.cf
       - master.cf=resources/postfix/master.cf
+    options:
+      disableNameSuffixHash: true
+  - name: postfix-maps
+    files:
       - transport=resources/postfix/transport
     options:
       disableNameSuffixHash: true