SELECT 
  * 
FROM 
  cscart_bm_containers as c 
  LEFT JOIN cscart_bm_grids as g ON g.container_id = c.container_id 
WHERE 
  1 
  AND g.container_id IN (1, 2, 15, 4) 
ORDER BY 
  g.order, 
  g.parent_id, 
  g.grid_id ASC

Query time 0.00076

JSON explain

{
  "query_block": {
    "select_id": 1,
    "read_sorted_file": {
      "filesort": {
        "sort_key": "g.`order`, g.parent_id, g.grid_id",
        "table": {
          "table_name": "g",
          "access_type": "range",
          "possible_keys": ["container_id"],
          "key": "container_id",
          "key_length": "3",
          "used_key_parts": ["container_id"],
          "rows": 18,
          "filtered": 100,
          "index_condition": "g.container_id in (1,2,15,4)"
        }
      }
    },
    "table": {
      "table_name": "c",
      "access_type": "eq_ref",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "3",
      "used_key_parts": ["container_id"],
      "ref": ["rfswire_cart.g.container_id"],
      "rows": 1,
      "filtered": 100
    }
  }
}