Add basic docker image / util script for building

This adds a Dockerfile which builds a docker image that contains everything you need to successfully run ./test/ci.sh.

There's also a script (./docker-build.sh) that runs a build inside the container (which has been uploaded to index.docker.io as joshuawarner32/avian-build).

I'll work on building extended images on top of this one that let you do cross-compile build and test runs (perhaps using qemu-user-static).