make docker image build work

- Depend on a python version Debian actually ships with in
  pyproject.toml and update the poetry.lock accordingly.
- Fix poetry bundle complaining about no python by installing
  python-is-python3.
  Also remove the python flag for poetry bundle then.

Dockerfile

@@ -1,12 +1,12 @@
 FROM docker.io/library/debian:12-slim AS builder
 RUN apt-get update && \
-    apt-get install --no-install-suggests --no-install-recommends --yes pipx
+    apt-get install --no-install-suggests --no-install-recommends --yes pipx python-is-python3
 ENV PATH="/root/.local/bin:${PATH}"
 RUN pipx install poetry
 RUN pipx inject poetry poetry-plugin-bundle
 WORKDIR /src
 COPY . .
-RUN poetry bundle venv --python=/usr/bin/python3 --only=main /venv
+RUN poetry bundle venv --only=main /venv
 
 FROM gcr.io/distroless/python3-debian12
 COPY --from=builder /venv /venv