mongodb names its secrets' owner: the image drops to its own user before it reads the password file

The official entrypoint re-executes itself as mongodb (uid 999) and only then reads
MONGO_INITDB_ROOT_PASSWORD_FILE, so a root-owned 0600 file is 'Permission denied' at line 83 and
the server never starts. secrets-owner is the mechanism ADR 0086 gives for exactly this.

modules/mongodb/module.json

@@ -41,6 +41,7 @@
     "root": "/var/lib/mongodb/root.secret",
     "broker": "/var/lib/mesh/mongodb/broker"
   },
+  "secrets-owner": "999:999",
   "resources": [
     {
       "id": "mesh-state",