如何在docker .env文件中使用环境变量？

我有一个缓存文件夹，路径存储在一个环境变量中： $LOCAL_CACHE (export LOCAL_CACHE=/home/me/path/to/any/cache/folder)

现在，我想在.env文件中附加一些其他文件夹：

我的.env文件：

LOCAL_COMPOSER_DIR=${LOCAL_CACHE}/composer
LOCAL_NPM_DIR=${LOCAL_CACHE}/npm
LOCAL_BOWER_DIR=${LOCAL_CACHE}/bower

我的docker_compose.yml看起来像这样：

version: '2'
services:
  composer:
    env_file: .env
    image: composer/composer
    volumes:
      - ./src:${APP_ROOT}
      - ${LOCAL_COMPOSER_DIR}:/composer
    working_dir: ${APP_ROOT}

当我启动服务时，docker-compose run composer instal. ...它停止并显示以下错误：ERROR: Named volume "$LOCAL_CACHE"/composer":/composer:rw" is used in service "composer" but no declaration was found in the volumes section.

我不确定，但是.env文件似乎不支持使用变量。