Split out the container build

.github/workflows/python_backend_tests.yaml

@@ -12,9 +12,9 @@ jobs:
         run: |
           python -m pip install --upgrade pip
           pip install -r tests/python_backend/requirements.txt
-      - name: Download container
+      - name: Build container
         run: |
-          docker pull moyix/triton_with_ft:22.09
+          docker compose build
       - name: Run tests
         run: pytest tests