configure composer to install vendor in custom path

This configuration is done using local environment variable (previous
config was overwritten by mistake)

.gitlab-ci.yml

@@ -5,6 +5,7 @@ image: registry.gitlab.com/chill-projet/chill-app/php-base-image:7.4
 cache:
   paths:
     - tests/app/vendor/
+    - /tmp/composer-cache
 
 before_script:
   # add extensions to postgres
@@ -34,6 +35,10 @@ variables:
   REDIS_HOST: redis
   REDIS_PORT: 6379
   REDIS_URL: redis://redis:6379
+  # change vendor dir to make the app install into tests/apps
+  COMPOSER_VENDOR_DIR: tests/app/vendor
+  # cache some composer data
+  COMPOSER_HOME: /tmp/composer-cache
 
 
 # Run our tests