Fix export stage

Since `pipx` didn't end up on the `PATH` (`ensurepath` only seems to update `.bashrc`) we were actually using the latest version of `poetry`. Avoid `pipx` all-together, using this installation method described in the poetry documentation.

Dockerfile

@@ -1,15 +1,14 @@
 FROM docker.io/python:3.11.10@sha256:b0c14c5ad67e76f603a72a165e89682b48898800b815e09f34c13ea87feb66d3 AS req
 
-RUN python3 -m pip install pipx && \
-  python3 -m pipx ensurepath
-
-RUN pipx install poetry==1.8.3 && \
-  pipx inject poetry poetry-plugin-export && \
-  pipx run poetry config warnings.export false
+ENV POETRY_HOME=/opt/poetry
+RUN <<EOF
+python3 -m venv $POETRY_HOME
+$POETRY_HOME/bin/pip install poetry==1.8.3
+EOF
 
 COPY ./poetry.lock ./poetry.lock
 COPY ./pyproject.toml ./pyproject.toml
-RUN pipx run poetry export --format requirements.txt --output requirements.txt
+RUN $POETRY_HOME/bin/poetry export --format requirements.txt --output requirements.txt
 
 FROM docker.io/python:3.11.10@sha256:b0c14c5ad67e76f603a72a165e89682b48898800b815e09f34c13ea87feb66d3