test(devcontainer): properly install PHP extensions for testing

.devcontainer/devcontainer.json

@@ -36,7 +36,10 @@
         }
     },
     "features": {
-        "ghcr.io/devcontainers-contrib/features/mocha:2": {}
+        "ghcr.io/devcontainers-contrib/features/mocha:2": {},
+        "ghcr.io/yassinedoghri/devcontainers/php-extensions-installer:1": {
+            "extensions": "gd sqlite3"
+        }
     },
     "forwardPorts": [
         8080