switch CI workflow to GitHub Actions

.github/workflows/ci.yml

@@ -0,0 +1,45 @@
+name: CI
+on:
+  push:
+    branches: ['**']
+    paths-ignore: ['*.adoc', 'docs/**']
+  pull_request:
+    branches: [master]
+    paths-ignore: ['*.adoc', 'docs/**']
+  #schedule:
+  #- cron: '30 2 * * MON'
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+jobs:
+  activate:
+    if: |
+      (github.event_name == 'schedule' && github.repository_owner == 'asciidoctor') ||
+      (github.event_name == 'push') ||
+      (github.event_name == 'pull_request' && !startsWith(github.head_ref, 'docs/'))
+    runs-on: ubuntu-latest
+    steps:
+    - name: Proceed
+      run: echo ok go
+  build:
+    needs: activate
+    strategy:
+      matrix:
+        os: [ubuntu-latest]
+        ruby: ['2.7']
+    runs-on: ${{ matrix.os }}
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v3
+    - name: Install Ruby (uses cached dependencies for non-scheduled build)
+      uses: ruby/setup-ruby@v1
+      with:
+        ruby-version: ${{ matrix.ruby }}
+        bundler-cache: ${{ github.event_name != 'schedule' }}
+    - name: Install dependencies (scheduled build only)
+      if: github.event_name == 'schedule'
+      run: |
+        bundle config --local path vendor/bundle
+        bundle --jobs 3 --retry 3
+    - name: Run tests
+      run: bundle exec ruby -w $(bundle exec ruby -e 'print File.join Gem.bindir, %q(rake)') spec