ci(github): add GitHub Actions CI workflow

Can be tested locally via the local-ci make target, which requires
Docker and act (https://github.com/nektos/act).

.github/workflows/ci.yml

@@ -0,0 +1,38 @@
+---
+name: CI
+on:
+  push:
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        php_version:
+          - "7.4"
+          - "7.3"
+          - "7.2"
+          - "7.1"
+    steps:
+      - uses: actions/checkout@v2
+      - uses: shivammathur/setup-php@v2
+        with:
+          php-version: ${{ matrix.php_version }}
+        env:
+          COMPOSER_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+      - name: Get composer cache directory
+        id: composer-cache
+        run: echo "::set-output name=dir::$(composer config cache-files-dir)"
+      - name: Cache composer dependencies
+        uses: actions/cache@v2
+        with:
+          path: ${{ steps.composer-cache.outputs.dir }}
+          key: ${{ runner.os }}-composer-${{ hashFiles('**/composer.lock') }}
+          restore-keys: ${{ runner.os }}-composer-
+      - name: Install dependencies
+        run: composer update
+      - name: Validate dependencies
+        run: composer validate
+      - name: Run tests
+        run: vendor/bin/phpunit --configuration tests/phpunit.xml

Makefile

@@ -11,5 +11,11 @@ phpunit-dep:
 		exit 1 \
 	)
 
+# Requires:
+# - Docker: https://docker.com
+# - act: https://github.com/nektos/act
+local-ci:
+	act -P ubuntu-latest=shivammathur/node:latest -W .github/workflows/ci.yml
+
 .SILENT:
-.PHONY: test phpunit-dep
+.PHONY: test phpunit-dep local-ci