demo-stb: Add .dockerignore to speed docker build

This speeds up the "Sending build context to Docker daemon" step and the
Dockerfile "ADD ." step substantially, when you have a local copy of the
assets (perhaps because you test the demo-stb directly in
chromium-browser on your host system). Note that the `docker build`
process will download the assets whether you have them on the host or
not -- this is to take advantage of docker's caching to speed `docker
build` when the only thing that has changed is the virtual-stb code
itself.