fix(ci): single RUN for deps + uv install on legacy DinD

Legacy docker cannot chain shell after heredoc and does not persist
/tmp between RUN steps. Use python -c and one RUN for requirements
generation and venv install.

.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}/${IMAGE_NAME}"
-          docker build --build-arg DEPS_LOCK_ID=4 -f buildctx/Dockerfile.ci \
+          docker build --build-arg DEPS_LOCK_ID=5 -f buildctx/Dockerfile.ci \
             -t "${IMAGE}:latest" -t "${IMAGE}:main-${SHORT}" buildctx
           docker push "${IMAGE}:latest"
           docker push "${IMAGE}:main-${SHORT}"