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/Makefile

@@ -0,0 +1,14 @@
+
+help:
+	cat Makefile
+run:
+	docker-compose up
+exec:
+	docker-compose exec handson-ml /bin/bash
+build: stop .FORCE
+	docker-compose build
+rebuild: stop .FORCE
+	docker-compose build --force-rm
+stop:
+	docker stop handson-ml || true; docker rm handson-ml || true;
+.FORCE: