Build docker images for releases.

It's handy to have jsonnet (and probably jsonnetfmt) packaged as docker images. At the moment bitnami make an image: https://hub.docker.com/r/bitnami/jsonnet/, but it would be good if this was done for each release using the release binaries automatically. I can make the config to build the images.

Presumably it makes sense to put them on docker hub so that people can easily pull them.

Someone could create a jsonnet organisation on docker hub, or else perhaps it makes sense for the images to live under the google organisation - although someone will have to get hold of credentials to push images to docker hub for that.