Merge pull request #1806 from ChrisPappalardo/pycon2017_sprints_docker

DOC: Add instructions for common Docker build failures

Dockerfile

@@ -21,7 +21,7 @@
 #
 #    docker exec -it zipline zipline run -f /projects/my_algo.py --start 2015-1-1 --end 2016-1-1 /projects/result.pickle
 #
-FROM python:2.7
+FROM python:3.5
 
 #
 # set up environment

Dockerfile-dev

@@ -3,6 +3,10 @@
 #
 #    docker build -t quantopian/ziplinedev -f Dockerfile-dev .
 #
+# Note: the dev build requires a quantopian/zipline image, which you can build as follows:
+#
+#    docker build -t quantopian/zipline -f Dockerfile
+#
 # To run the container:
 #
 #    docker run -v /path/to/your/notebooks:/projects -v ~/.zipline:/root/.zipline -p 8888:8888/tcp --name ziplinedev -it quantopian/ziplinedev