Docker build simplification

The attached patch simplifies the Docker container build so that it can be
done entirely using "docker build" in the root of a source tree. It also
enables building the container from a tarball, not just a git checkout.
This does mean that container builds may now also include any garbage in
the source tree from which they're built, but conversely offers the benefit
of making it easy to build a container with code changes in it without
having to commit first.

Akshay, can you look it over to make sure I haven't done anything obviously
stupid? Feedback from others is also welcome of course.

--
Dave Page
Blog: http://pgsnake.blogspot.com
Twitter: @pgsnake

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