Dockerfile: lock poetry installation to v2.0.1

This avoids the docker image builders breaking with:
ERROR [linux/amd64 builder 15/17] RUN poetry export -o requirements.txt --without-hashes
0.780 pyproject.toml changed significantly since poetry.lock was last generated. Run Resolving dependencies... to fix the lock file.

This occurred when the default installation version changed underneath us.

Changelog-None

Dockerfile

@@ -90,7 +90,8 @@ RUN apt-get update -qq && \
         tclsh
 
 ENV PATH="/root/.local/bin:$PATH" \
-    PYTHON_VERSION=3
+    PYTHON_VERSION=3 \
+    POETRY_VERSION=2.0.1
 RUN curl -sSL https://install.python-poetry.org | python3 - && \
     poetry self add poetry-plugin-export
 RUN mkdir -p /root/.venvs && \