常連

### 前提・実現したいこと

RailsのアプリケーションでCircleCIを使おうと思ったのですが、CI側でRSpecを実行する際にデータベースのセットアップでエラーが出ました。
.circleci/config.ymlは https://circleci.com/docs/2.0/language-ruby/ を元に作りました。

### 発生している問題・エラーメッセージ

```
#!/bin/bash -eo pipefail
# bundle exec rails db:create
# bundle exec rails db:schema:load --trace
could not translate host name "db" to address: Name or service not known
Couldn't create 'book_manager_test' database. Please check your configuration.
rails aborted!
PG::ConnectionBad: could not translate host name "db" to address: Name or service not known
```

### 該当のソースコード
.circleci/config.yml
```yml
version: 2.1 # Use 2.1 to enable using orbs and other features.

# Declare the orbs that we'll use in our config.
# read more about orbs: https://circleci.com/docs/2.0/using-orbs/
orbs:
  ruby: circleci/ruby@1.0
  node: circleci/node@2

jobs:
  build: # our first job, named "build"
    docker:
      - image: cimg/ruby:2.6.6-node # use a tailored CircleCI docker image.
    steps:
      - checkout # pull down our git code.
      - ruby/install-deps # use the ruby orb to install dependencies
      # use the node orb to install our packages
      # specifying that we use `yarn` and to cache dependencies with `yarn.lock`
      # learn more: https://circleci.com/docs/2.0/caching/
      - node/install-packages:
          pkg-manager: yarn
          cache-key: 'yarn.lock'

  rubocop:
    docker:
      - image: cimg/ruby:2.6.6-node # use a tailored CircleCI docker image.
        environment:
          RAILS_ENV: test
          POSTGRES_HOST: 127.0.0.1
      - image: circleci/postgres:9.5-alpine
        environment: # add POSTGRES environment variables.
          POSTGRES_USER: circleci-demo-ruby
          POSTGRES_DB: rails_blog_test
          POSTGRES_PASSWORD: ''
    steps:
      - checkout
      - ruby/install-deps
      - node/install-packages:
          pkg-manager: yarn
          cache-key: 'yarn.lock'
      # Rubocop
      - run:
          name: Rubocop
          command: bundle exec rubocop

  test: # our next job, called "test"
    # we run "parallel job containers" to enable speeding up our tests;
    # this splits our tests across multiple containers.
    # parallelism: 3
    # here we set TWO docker images.
    docker:
      - image: cimg/ruby:2.6.6-node # this is our primary docker image, where step commands run.
      - image: circleci/postgres:9.5-alpine
        environment: # add POSTGRES environment variables.
          POSTGRES_USER: postgres
          POSTGRES_DB: rails_blog_test
          POSTGRES_PASSWORD: password
    # environment variables specific to Ruby/Rails, applied to the primary container.
    environment:
      BUNDLE_JOBS: '3'
      BUNDLE_RETRY: '3'
      PGHOST: 127.0.0.1
      PGUSER: postgres
      PGPASSWORD: password
      RAILS_ENV: test
    # A series of steps to run, some are similar to those in "build".
    steps:
      - checkout
      - ruby/install-deps
      - node/install-packages:
          pkg-manager: yarn
          cache-key: 'yarn.lock'
      # Here we make sure that the secondary container boots
      # up before we run operations on the database.
      - run:
          name: Wait for DB
          command: dockerize -wait tcp://localhost:5432 -timeout 1m
      - run:
          name: Database setup
          command: |
            bundle exec rails db:create
            bundle exec rails db:schema:load --trace
      # Run rspec in parallel
      - ruby/rspec-test

# We use workflows to orchestrate the jobs that we declared above.
workflows:
  version: 2
  build_and_test: # The name of our workflow is "build_and_test"
    jobs: # The list of jobs we run as part of this workflow.
      - build # Run build first.
      - rubocop:
          requires:
            - build
      - test: # Then run test,
          requires: # Test requires that build passes for it to run.
            - build # Finally, run the build job.

```
config/database.yml
```yml
default: &default
  adapter: postgresql
  encoding: unicode
  host: db
  username: postgres
  password: password
  pool: 5

development:
  <<: *default
  database: book_manager_development

test:
  <<: *default
  database: book_manager_test

```

```Dockerfile
FROM ruby:2.6

RUN curl -sS https://dl.yarnpkg.com/debian/pubkey.gpg | apt-key add - \
    && echo "deb https://dl.yarnpkg.com/debian/ stable main" | tee /etc/apt/sources.list.d/yarn.list

RUN apt-get update -qq && apt-get install -y nodejs postgresql-client yarn

RUN mkdir /book_manager
WORKDIR /book_manager
COPY Gemfile /book_manager/Gemfile
COPY Gemfile.lock /book_manager/Gemfile.lock
RUN bundle install
COPY . /book_manager

# Add a script to be executed every time the container starts.
COPY entrypoint.sh /usr/bin/
RUN chmod +x /usr/bin/entrypoint.sh
ENTRYPOINT ["entrypoint.sh"]
EXPOSE 3000

# Start the main process.
CMD ["rails", "server", "-b", "0.0.0.0"]

```
docker-compose.yml
```yml
version: '3'
services:
  web:
    build: .
    command: bash -c "rm -f tmp/pids/server.pid && bundle exec rails s -p 3000 -b '0.0.0.0'"
    volumes:
      - .:/book_manager
    ports:
      - '3000:3000'
    depends_on:
      - db
  db:
    image: postgres
    volumes:
      - ./tmp/db:/var/lib/postgresql/data
    environment:
      - POSTGRES_PASSWORD=password
    ports:
      - '5432:5432'
```

### 試したこと

database.ymlを
```yml
test:
  <<: *default
  database: book_manager_test
  host: localhost
```
のように`host: localhost`を追加するとCI側では正しくテストが走るようになりましたが、
この状態で手元でRSpecを実行すると以下のエラーが出ます。
```

PG::ConnectionBad:
  could not connect to server: Connection refused
  	Is the server running on host "localhost" (127.0.0.1) and accepting
  	TCP/IP connections on port 5432?
  could not connect to server: Cannot assign requested address
  	Is the server running on host "localhost" (::1) and accepting
  	TCP/IP connections on port 5432?
```

### 補足情報（FW/ツールのバージョンなど）
Ruby 2.6.6
Rails 6.0.3

	@@ -1 +1 @@
1	- Rails + CircleCIで~~PG::Conn~~ec~~tion~~B~~ad: could not translate host name "db" to address~~が出る
1	+ Rails + CircleCIでRSpecを実行しようとするとDBのエラーが出る

	@@ -1 +1 @@
1	- Rails + CircleCIでcould not translate host name "db" to address~~: Name or service not known~~
1	+ Rails + CircleCIでPG::ConnectionBad: could not translate host name "db" to addressが出る

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