circleciでrspecを実行すると、Webpacker can't find applicationのエラーが出た

## ローカルでは通っていたテストがcircleciで実行すると、Webpacker can't find applicationのエラーが出て、解決できたのでメモとして記録

## 参考URL
https://qiita.com/Shantti-Y/items/8ddc1aee3b854ee7877c


## rspecのエラー全文
```ruby:
Failure/Error: <%= javascript_pack_tag 'application', 'data-turbolinks-track': 'reload' %>
       
       ActionView::Template::Error:
         Webpacker can't find application in /home/circleci/sample_app/public/packs-test/manifest.json. Possible causes:
         1. You want to set webpacker.yml value of compile to true for your environment
            unless you are using the `webpack -w` or the webpack-dev-server.
         2. webpack has not yet re-run to reflect updates.
         3. You have misconfigured Webpacker's config/webpacker.yml file.
         4. Your webpack configuration is not creating a manifest.
         Your manifest contains:
         {
         }

```
## 原因
circleciの環境の中でyarnが必要にも関わらずインストールをせずに、**- run: bundle exec bin/webpack**でビルドしようとしていたから、エラーが発生していた



###.circleci/config.yalの中身（エラー発生時）
```ruby:
version: 2.1
orbs:
  ruby: circleci/ruby@0.1.2

jobs:
  build:
    docker:
      # specify the version you desire here
      - image: circleci/ruby:2.5.3-stretch-node
        environment:
          - RAILS_ENV: 'test'
      - image: circleci/mysql:8.0
        name: "db"
        command: mysqld --default-authentication-plugin=mysql_native_password
        environment:
          - MYSQL_ROOT_PASSWORD: password

      # Specify service dependencies here if necessary
      # CircleCI maintains a library of pre-built images
      # documented at https://circleci.com/docs/2.0/circleci-images/
      # - image: circleci/postgres:9.4

    working_directory: ~/sample_app

    steps:
      - checkout

      # Download and cache dependencies
      - restore_cache:
          keys:
            - v1-dependencies-{{ checksum "Gemfile.lock" }}
            # fallback to using the latest cache if no exact match is found
            - v1-dependencies-

      - run: bundle install --jobs=4 --retry=3 --path vendor/bundle
      - save_cache:
          paths:
            - ./vendor/bundle
          key: v1-dependencies-{{ checksum "Gemfile.lock" }}

      # Database setup
      - run: bundle exec rake db:create
      - run: bundle exec rake db:schema:load

      - run: bundle exec bin/webpack

# rspec
      # run tests!
      - run:
          name: run tests
          command: |

            mkdir -p /tmp/test-results
            TEST_FILES="$(circleci tests glob "spec/**/*_spec.rb" | \
              circleci tests split --split-by=timings)"


            bundle exec rspec \
              --format progress \
              --format RspecJunitFormatter \
              --out /tmp/test-results/rspec.xml \
              --format progress \
              $TEST_FILES

      # collect reports
      - store_test_results:
          path: /tmp/test-results
      - store_artifacts:
          path: /tmp/test-results
          destination: test-results
```


## 解決方法
先にyarnをインストールしてから、実行してやるとエラーは消えた

```ruby:
 +    - run:
 +        name: yarn Install
 +        command: yarn install
      - run: bundle exec bin/webpack
```


## 反省
全般的にやけど、理解もせずに使ってしまっているからエラーの解決に苦戦してしまう。