feat(ci): use lthn/build:php container image

Replace setup-php action with pre-built lthn/build:php-* container
images. Eliminates ~50s PHP setup overhead per matrix job.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.forgejo/workflows/php-test.yml

@@ -10,10 +10,6 @@ on:
         description: PHP versions to test (JSON array)
         type: string
         default: '["8.3", "8.4"]'
-      extensions:
-        description: PHP extensions to install
-        type: string
-        default: dom, curl, libxml, mbstring, zip, pcntl, pdo, sqlite, pdo_sqlite
       coverage:
         description: Generate coverage report
         type: boolean
@@ -27,6 +23,8 @@ jobs:
   test:
     name: PHP ${{ matrix.php }}
     runs-on: ubuntu-latest
+    container:
+      image: lthn/build:php-${{ matrix.php }}
 
     strategy:
       fail-fast: true
@@ -36,13 +34,6 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
-      - name: Setup PHP
-        uses: https://github.com/shivammathur/setup-php@v2
-        with:
-          php-version: ${{ matrix.php }}
-          extensions: ${{ inputs.extensions }}
-          coverage: pcov
-
       - name: Checkout dependencies
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}