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

Replace setup-php action with pre-built container.
Eliminates ~50s setup overhead per matrix job.

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

.forgejo/workflows/ci.yml

@@ -10,6 +10,8 @@ jobs:
   test:
     name: PHP ${{ matrix.php }}
     runs-on: ubuntu-latest
+    container:
+      image: lthn/build:php-\${{ matrix.php }}
 
     strategy:
       fail-fast: true
@@ -19,13 +21,6 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
-      - name: Setup PHP
-        uses: https://github.com/shivammathur/setup-php@v2
-        with:
-          php-version: ${{ matrix.php }}
-          extensions: dom, curl, libxml, mbstring, zip, pcntl, pdo, sqlite, pdo_sqlite
-          coverage: pcov
-
       - name: Install dependencies
         run: composer install --prefer-dist --no-interaction --no-progress