fix: consolidate to Python 3.12 in pythonr image

Two Python versions were present in the pythonr image:
- 3.12 at /usr/local/bin/python (intentional, from base image)
- 3.11 at /usr/bin/python3 (unintentional, pulled in as a transitive
  apt dependency when pak/sf resolves GDAL system libs on Debian 12)

Fixes:
- pythonr/Dockerfile: register /usr/local/bin/python as the
  higher-priority python3 alternative via update-alternatives, so
  both 'python' and 'python3' resolve to 3.12 without breaking
  system packages that depend on the binary
- python/Dockerfile: add python.defaultInterpreterPath to the
  devcontainer.metadata LABEL so VSCode selects 3.12 automatically
  in any derived devcontainer

Closes #132

Author: Robinlovelace

python/Dockerfile

@@ -3,7 +3,7 @@ FROM ${PYTHON_BASE_IMAGE}
 
 ARG QUARTO_VERSION=1.9.37
 
-LABEL devcontainer.metadata='[{"customizations":{"vscode":{"extensions":["quarto.quarto","ms-python.python","ms-toolsai.jupyter"]}}}]'
+LABEL devcontainer.metadata='[{"customizations":{"vscode":{"extensions":["quarto.quarto","ms-python.python","ms-toolsai.jupyter"],"settings":{"python.defaultInterpreterPath":"/usr/local/bin/python"}}}}]'
 
 # Install dependencies, git, and gh cli
 RUN apt-get update && apt-get install -y --no-install-recommends \

pythonr/Dockerfile

@@ -17,6 +17,10 @@ ENV PATH="/opt/R/${R_VERSION}/bin:${PATH}"
 ENV R_PKG_TYPE=binary
 ENV RETICULATE_PYTHON="/usr/local/bin/python"
 
+# Ensure python3 resolves to the 3.12 interpreter from the base image,
+# not the Debian system Python 3.11 pulled in as a transitive dependency.
+RUN update-alternatives --install /usr/bin/python3 python3 /usr/local/bin/python 10
+
 RUN mkdir -p /etc/R && \
     echo "options(repos = c(CRAN = 'https://packagemanager.posit.co/cran/__linux__/bookworm/latest'))" > /etc/R/Rprofile.site && \
     mkdir -p /root && \