Running the Elasticsearch Docker image with a different GID is
possible but trappy, since at present all the ES files are only
readable by the user and group. This PR documents a Docker CLI flag
that fixes this situation, by ensuring the container user is added
to the default group (which is `root`, GID 0).
I also added a test for this case, and refactored the Docker tests
to use a builder pattern for constructing the `docker run` command.
The existing code was becoming unwieldy and hard to change.
a32a0986c3