Use composer:2 container in Gitea workflows

.gitea/workflows/tests.yaml

@@ -10,15 +10,9 @@ jobs:
   tests:
     runs-on: ubuntu-latest
     container:
-      image: php:7.4-cli
+      image: composer:2
 
     steps:
-      - name: Install system dependencies
-        run: |
-          apt-get update -qq
-          apt-get install -y -qq git unzip curl
-          curl -sS https://getcomposer.org/installer | php -- --install-dir=/usr/local/bin --filename=composer
-
       - uses: actions/checkout@v3
 
       - name: Install Composer dependencies