Use prebuilt Docker image for Gitea workflows

.gitea/workflows/check.yml

@@ -8,20 +8,16 @@ on:
 jobs:
   check:
     runs-on: ubuntu-latest
+    container:
+      image: daily-blackout-check-runner:latest
+    env:
+      PYTHONPATH: src
     steps:
       - name: Check out repository
         uses: https://gitea.com/actions/checkout@v4
         with:
           fetch-depth: 0
 
-      - name: Set up Python
-        uses: https://gitea.com/actions/setup-python@v5
-        with:
-          python-version: "3.13"
-
-      - name: Install project
-        run: python -m pip install .
-
       - name: Check outages and persist state
         env:
           BARGHEMAN_TOKEN: ${{ secrets.BARGHEMAN_TOKEN }}