MANGO-1216 feat(GHA/CI/CD): move to GHA

One of the strategic goals is to move away from CircleCI into GitHub
Actions.

See: https://github.com/blinkist/backend-starter-kit/blob/main/docs/adrs/010_github_actions.md

.github/workflows/ci.yml

@@ -0,0 +1,40 @@
+name: CI
+
+on: [ push ]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    services:
+      postgres:
+        image: postgres
+        env:
+          POSTGRES_DB: myapp_test
+          POSTGRES_USER: postgres
+          POSTGRES_PASSWORD: postgres
+        ports:
+          - 5432:5432
+        options: >-
+          --health-cmd pg_isready
+          --health-interval 10s
+          --health-timeout 5s
+          --health-retries 5
+
+    steps:
+    - uses: actions/checkout@v4
+
+    - name: Set up Ruby
+      uses: ruby/setup-ruby@v1
+      with:
+        ruby-version: 2.7.3
+        bundler-cache: true
+
+    - name: Bundle Install
+      run: bundle install --jobs=4 --retry=3 --path=vendor/bundle
+
+    - name: Run RSpec
+      env:
+        RAILS_ENV: test
+        RACK_ENV: test
+      run: bundle exec rspec --format progress spec