added -f flag to all conda clean commands

This commit adds the additional `-f` force command to all uses of `conda
clean --all` through the repo. Size should be smaller, but still testing
if anything breaks. See issue #861.

pyspark-notebook/Dockerfile

@@ -45,6 +45,6 @@ USER $NB_UID
 
 # Install pyarrow
 RUN conda install --quiet -y 'pyarrow' && \
-    conda clean --all -y && \
+    conda clean --all -f -y && \
     fix-permissions $CONDA_DIR && \
     fix-permissions /home/$NB_USER