Move the image we use to generate the docker official-images changes into flink-docker

Before the docker official image was repatriated into Apache Flink we used a docker image that contained the scripts to generate the release here: https://github.com/apache/flink-docker/blob/e0107b0f85d9b3630db39568729e36408cdc7b78/generate-stackbrew-library-docker.sh#L5.

docker run --rm
    --volume ~/projects/docker-flink:/build
    plucas/docker-flink-build
    /build/generate-stackbrew-library.sh > ~/projects/official-images  /library/flink

Notice that this docker image tool 'plucas/docker-flink-build' is not part of upstream Flink so we need to move it there into some sort of tools section in the flink-docker repo or document an alternative to it.