fix(ci): store requirements.txt under /app not /tmp

DinD legacy builder does not persist /tmp across RUN steps, so uv pip
install could not find requirements.txt on the next layer.

.forgejo/workflows/image.yaml

@@ -65,7 +65,7 @@ jobs:
           export PATH="${HOME}/bin:${PATH}"
           echo "${REGISTRY_TOKEN}" | docker login "${REGISTRY}" -u "${REGISTRY_USER}" --password-stdin
           IMAGE="${REGISTRY}/${IMAGE_NAME}"
-          docker build --build-arg DEPS_LOCK_ID=3 -f buildctx/Dockerfile.ci \
+          docker build --build-arg DEPS_LOCK_ID=4 -f buildctx/Dockerfile.ci \
             -t "${IMAGE}:latest" -t "${IMAGE}:main-${SHORT}" buildctx
           docker push "${IMAGE}:latest"
           docker push "${IMAGE}:main-${SHORT}"