update db init script

mongo-init.sh

@@ -2,7 +2,7 @@ set -eu
 mongo -- "$dbname" <<EOF
     var rootUser = '$MONGO_INITDB_ROOT_USERNAME';
     var rootPassword = '$MONGO_INITDB_ROOT_PASSWORD';
-    var admin = db.getSiblingDB('$MONGO_INITDB_DATABASE');
+    var admin = db.getSiblingDB('$dbname');
     admin.auth(rootUser, rootPassword);
     var user = '$dbuser';
     var passwd = '${dbpass-}' || user;