allow configuring python version using buildarg

base-notebook/Dockerfile

@@ -63,6 +63,7 @@ RUN echo "auth requisite pam_deny.so" >> /etc/pam.d/su && \
 
 USER $NB_UID
 WORKDIR $HOME
+ARG PYTHON_VERSION=default
 
 # Setup work directory for backward-compatibility
 RUN mkdir /home/$NB_USER/work && \
@@ -82,9 +83,10 @@ RUN cd /tmp && \
     $CONDA_DIR/bin/conda config --system --prepend channels conda-forge && \
     $CONDA_DIR/bin/conda config --system --set auto_update_conda false && \
     $CONDA_DIR/bin/conda config --system --set show_channel_urls true && \
+    if [ ! $PYTHON_VERSION = 'default' ]; then conda install --yes python=$PYTHON_VERSION; fi && \
+    conda list python | grep '^python ' | tr -s ' ' | cut -d '.' -f 1,2 | sed 's/$/.*/' >> $CONDA_DIR/conda-meta/pinned && \
     $CONDA_DIR/bin/conda install --quiet --yes conda && \
     $CONDA_DIR/bin/conda update --all --quiet --yes && \
-    conda list python | grep '^python ' | tr -s ' ' | cut -d '.' -f 1,2 | sed 's/$/.*/' >> $CONDA_DIR/conda-meta/pinned && \
     conda clean --all -f -y && \
     rm -rf /home/$NB_USER/.cache/yarn && \
     fix-permissions $CONDA_DIR && \