Composer 安装失败，使用 CircleCI 和 Laravel 6

为了学习 CI，我在 Homestead 内部启动了一个新的 Laravel 项目。

我的circleci config.yml 如下所示

# PHP CircleCI 2.0 configuration file
#
# Check https://circleci.com/docs/2.0/language-php/ for more details
#
version: 2
jobs:
  build:
    docker:
      # Specify the version you desire here
      - image: circleci/php:7.3-stretch-node-browsers

      # Specify service dependencies here if necessary
      # CircleCI maintains a library of pre-built images
      # documented at https://circleci.com/docs/2.0/circleci-images/
      # Using the RAM variation mitigates I/O contention
      # for database intensive operations.
      # - image: circleci/mysql:5.7-ram
      # 
      # - image: redis:3

    steps:
      - checkout

      - run: sudo apt update && sudo apt install zlib1g-dev libsqlite3-dev
      - run: sudo docker-php-ext-install zip

      # Download and cache dependencies

      # composer cache
      - restore_cache:
          keys:
          # "composer.lock" can be used if it is committed to the repo
          - v1-dependencies-{{ checksum "composer.json" }}
          # fallback to using the latest cache if no exact match is found
          - v1-dependencies-

      - run: composer install -n --prefer-dist

      - save_cache:
          key: composer-v1-{{ checksum "composer.lock" }}
          paths:
            - vendor

      # node cache

      - restore_cache:
          keys:
            - node-v4-{{ checksum "package.json" }}
            - node-v4-
      - run: npm install
      - save_cache:
          key: node-v4-{{ checksum "package.json" }}
          paths:
            - node_modules
            - ~/.yarn

      # prepare the database
      - run: touch storage/testing.sqlite
      - run: php artisan migrate --env=testing --database=sqlite_testing --force

      # run tests with phpunit or codecept
      #- run: ./vendor/bin/phpunit
      # this example uses codecept but you're not limited to it
      - run: ./vendor/bin/codecept build
      - run: ./vendor/bin/codecept run --xml result.xml
      - store_test_results:
          path: tests/_output
      - store_artifacts:
          path: tests/_output

当 CircleCI 到达

- run: composer install -n --prefer-dist

我收到以下错误消息：

尝试更新丢失的包仍然不起作用。

添加所需的存储库ppa:ondrej/php 会产生更多错误。

在本地，Laravel 位于 Homestead 内，PHP 版本为PHP 7.3.2-3

配置文件是来自https://raw.githubusercontent.com/CircleCI-Public/circleci-demo-php-laravel/circleci-2.0/.circleci/config.yml的示例配置

我的问题是，我该如何解决这个问题，以及如何将我自己的 config.yml 文件放在一起以适用于我的特定项目？