[tech debt] Cleaning up the Dockerfile

[MikeTheCanuck]

On the basis of PR #74, I'm now ready to cleanup tech debt in the build process and rely on the Travis build status to confirm I haven't broken anything (via the basic functional tests).

This cleanup removes three things:

1. The now-redundant download of CSV data to the Docker image. @mxmoss confirmed for me (again, due to my poor memory) that as of a week ago, the Django APIs were all relying on the external database rather than imported CSVs to construct their data responses.
2. The unreliable python:latest image, that @tedbrunner was unable to download when building the Docker container locally [while testing PR Travis runs automated tests! #74] on his Linux computer - he received an error "UNAUTHORIZED - authentication required" at step 1 of the docker container build, which corresponds to the error I see when I browse to the URL specified in the error output.
3. Also, trivially it is unnecessary to instruct Docker to RUN mkdir /code when the next statement is WORKDIR /code - according to Docker docs, the WORKDIR instruction will always create the target directory if it doesn't already exist.

[jimtyhurst]

I used start-proj.sh successfully with the Dockerfile set for FROM python:3.6.0. Is there any reason to stay with 3.5.3? There is nothing that we require in 3.6.0, but there are a couple of good enhancements in functionality and performance (https://docs.python.org/3/whatsnew/3.6.html).

[MikeTheCanuck]

The only reason I didn't immediately switch to the 3.6 container image as our baseline was because @tedbrunner had experienced repeatable failures when trying to run the test-proj.sh -l on his Linux machine.

I've had no such failures on my Mac.

When you tested Jim, was that on a Mac or a Linux box?

[jimtyhurst]

I tested with MacBook Pro running MacOS 10.12.3 (Sierra), using Python 3.6.0 in my virtualenv environment budget_venv. If other environments have problems with the python:latest = python:3.6.0 image, then that is a good enough reason to stay with python:3.5 Docker image.

[MikeTheCanuck]

Thanks Jim, sounds good - I've filed Issue #85 to capture this as a future issue to investigate, when/if needed.