build: add gitlab ci demo

.gitlab-ci.yml

+create-file:
+  stage: build
+  image: python
+  tags:
+    - gpu
+  script:
+    - |
+      cat <<EOF > test.py
+      with open('output.txt', 'w') as f:
+        f.write('hello world!\\n')
+      EOF
+    - python test.py
+  artifacts:
+    paths:
+      - output.txt
+
+test-file-output:
+  stage: test
+  script:
+    - grep -q "hello world!" output.txt
+  dependencies:
+    - create-file
+
+deploy-output:
+  stage: deploy
+  script:
+    - cp output.txt release.txt
+  artifacts:
+    paths:
+      - release.txt
+  dependencies:
+    - create-file

src/.gitlab-ci.yml

+../.gitlab-ci.yml
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