ci: change execution image

.gitlab-ci.yml

-image: php:latest
+image: composer
 
 cache:
     paths:
@@ -7,9 +7,9 @@ cache:
 build:
     stage: build
     script:
-        - ./tools/composer install
+        - composer install
 
 test:
     stage: test
     script:
-        - ./tools/composer test
+        - composer test