Run Postgres on GitHub Action

For some projects, like Phoenix project, we need a database for testing. Good news is setting it up is simple:

jobs:
  build:
    name: Build and test
    runs-on: ubuntu-latest
    services:
      postgres:
	image: postgres:11
	env:
	  POSTGRES_USER: postgres
	  POSTGRES_PASSWORD: postgres
	  POSTGRES_DB: my_app_test
	ports:
	  - 5432:5432
	options: --health-cmd pg_isready --health-interval 10s --health-timeout 5s --health-retries 5

This is all we need.