use pre-built dervish-ci image with pytest; no pip install in pipeline

.woodpecker.yml

@@ -3,7 +3,8 @@ when:
 
 steps:
   test:
-    image: python:3.11-slim
+    image: forgejo.corentic.eu/tobi/dervish-ci:latest
+    environment:
+      PYTHONPATH: .
     commands:
-      - pip install --no-cache-dir --editable . pytest
       - python -m pytest tests/ -v

Dockerfile.ci

@@ -0,0 +1,2 @@
+FROM python:3.11-slim
+RUN pip install --no-cache-dir pytest