PG::ConnectionBad: could not translate host name "db" to address: Temporary failure in name resolution

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こんなエラーが発生

PG::ConnectionBad:
  could not translate host name "db" to address: Temporary failure in name resolution

むむむ。。
データベースに接続できない。。

色々と調べた結果、以下を参考にし、修正しました。
https://qiita.com/kajirikajiri/items/bf0ead854bba58131040
https://stackoverflow.com/questions/3582552/postgresql-connection-url/20722229#20722229
Postgresqlのポートを指定し、DATABASE_URLを明示しました。

修正前

.github/workflows/ruby.yml

name: Ruby

on:
  pull_request:
    branches:
    - master


jobs:
  run_spec:
    name: Run spec

    runs-on: ubuntu-latest

    steps:
    - uses: actions/checkout@v1 
    - name: Set up Ruby 2.6
      uses: actions/setup-ruby@v1 # https://github.com/actions/setup-ruby/blob/master/action.yml
      with:
        ruby-version: 2.6.x
    - name: Test with Rspec
      run: |
        gem install bundler
        bundle install --path vendor/bundle --quiet --jobs 4 --retry 3
        bundle exec rspec

修正後

.github/workflows/ruby.yml

name: Ruby

on:
  pull_request:
    branches:
    - master

jobs:
  run_spec:
    name: Run spec

    runs-on: ubuntu-latest

    services:
      postgres:
        image: postgres:10.3
        ports: ["5432:5432"]
        options: --health-cmd pg_isready --health-interval 10s --health-timeout 5s --health-retries 5
    steps:
    - uses: actions/checkout@v1 
      uses: actions/setup-ruby@v1 
      with:
        ruby-version: 2.6.x
    - name: Test with Rspec
      env:
        RAILS_ENV: test
        DATABASE_URL: postgresql://postgres@localhost:5432/postgres?encoding=utf8&pool=5&timeout=5000
      run: |
        gem install bundler
        bundle install --path vendor/bundle --quiet --jobs 4 --retry 3
        bundle exec rspec

無事に成功しました！