Update .gitlab-ci.yml file

.gitlab-ci.yml

@@ -3,7 +3,7 @@ stages:          # List of stages for jobs, and their order of execution
 
 build-job:       # This job runs in the build stage, which runs first.
   stage: build
-  # image: python:slim
+  image: python:slim
   script:
     - echo "Compiling the code..."
     - pip install -U twine