Add PHP 8 support

.github/workflows/buildcheck.yaml

@@ -1,10 +1,8 @@
 name: buildcheck
-
-on:
-  push:
+"on":
+  push: null
   schedule:
     - cron: 0 10 6 * *
-
 jobs:
   build:
     runs-on: ubuntu-latest
@@ -19,32 +17,25 @@ jobs:
           - "8.3"
           - "8.4"
           - "8.5"
+          - "8"
         composer:
           - ""
           - "--prefer-lowest"
-
     steps:
       - uses: actions/checkout@v1
-
       - name: Create Docker Container
         run: |
           docker build . -t ci-image --build-arg PHP_VERSION=${{ matrix.php }}
           docker run --interactive --detach --volume ${{ github.workspace }}:/app --name ci ci-image
-
       - name: Install Dependencies
         run: docker exec ci composer update --no-interaction --no-ansi --prefer-dist ${{ matrix.composer }}
-
       - name: PHPUnit
         run: docker exec ci vendor/bin/phpunit
-
       - name: PHPStan
         run: docker exec ci vendor/bin/phpstan analyse --level=max src tests
-
       - name: Coding Standards
         run: docker exec ci vendor/bin/phpcs
-
       - name: Check Dependencies
         run: docker exec ci vendor/bin/composer-require-checker
-
       - name: Composer Validate
         run: docker exec ci composer validate --strict