Fix: Use composer:1 (PHP 7.4) instead of composer:2.0 (PHP 8.0)

Previous attempt used composer:2.0 which ships with PHP 8.0.6, but Laravel 7.x
requires PHP ^7.2.5. composer:1 provides PHP 7.4 which is the correct version
for this Laravel application.

.drone.yml

@@ -5,7 +5,7 @@ name: default
 steps:
   # Step 1: Install dependencies and run tests
   - name: test
-    image: composer:2.0
+    image: composer:1
     commands:
       - composer install --prefer-dist --no-progress --no-interaction
       - composer validate --strict