Merge pull request #149 from cyber-dojo/upgrade-docker-base-image

Upgrade docker base image.

- Rack::Deflater no longer works. For now I've commented this out since the amount of bytes returned by a cyber-dojo.sh runner call is not large.
- Removed docker-system-prune from bash script so docker layer caching works when building locally.
- In Dockerfile, moved source into /runner/source so that runner/test and runner/source are siblings. This allows runner/source to be overlayed in the docker-compose.yml file which gives a faster dev loop.
- Tweak container-properties-test to only check owners permission on /sandbox tmpfs.