docker: Fix `make docker-publish` and make more flexible

This allows you to customise the prefix and tag of the docker build
allowing publishing docker images to the docker hub under your username.
e.g.:

    make docker-publish docker_prefix=wmanley/ docker_tag=next
docker: Use stbtester as the docker prefix rather than stb_tester

I have registered the stbtester organisation on the docker registry.
Underscores are not allowed in the names.