Cleanup runner before building the backend docker image

.gitlab-ci.yml

@@ -208,6 +208,7 @@ build_backend:
   before_script:
     - docker login -u $CI_REGISTRY_USER -p $CI_REGISTRY_PASSWORD $CI_REGISTRY
   script:
+    - docker system prune -f
     - cd api
     - docker build -t $CI_REGISTRY_IMAGE:$CI_COMMIT_REF_NAME .
     - docker push $CI_REGISTRY_IMAGE:$CI_COMMIT_REF_NAME