Update .gitlab-ci.yml file

.gitlab-ci.yml

+image: python3.9
+
+stages:
+  - pipeline
+
+before_script:
+  - pip3 install -r requirements.txt
+
+pipeline:
+  stage: pipeline
+  script: 
+    - echo "build stage"
+    - jupyter nbconvert --to script app.ipynb
+    - if [ ! -f app.py ]; then echo "Conversion failed, exiting pipeline"; exit 1; fi
+    - echo "build completed"
+    - echo "Starting Flask"
+    - nohup python3 app.py &
+    - sleep 5
+    - echo "App should have started in the background"