Docker environment enhancements

- rearranged a Dockerfile to allow for incremental build
- switched running from root to "default new user"
- added the (easy to opt-out) configuration to use blank password
- added python-graphviz which enables DT visualization in notebooks
- added nbdime for "sensible notebook comparison"
- added custom command to "nbdiff" a notebook with its checkpointed version
- added simple README.md

docker/bashrc

@@ -0,0 +1,12 @@
+alias ll="ls -l"
+
+nbd() {
+	DIRNAME=$(dirname "$1")
+	BASENAME=$(basename "$1" .ipynb)
+
+	WORKING_COPY=$DIRNAME/$BASENAME.ipynb
+	CHECKPOINT_COPY=$DIRNAME/.ipynb_checkpoints/$BASENAME-checkpoint.ipynb
+
+	# echo "How change $CHECKPOINT_COPY into $WORKING_COPY"
+	nbdiff "$CHECKPOINT_COPY" "$WORKING_COPY"
+}