What does "set --" do in this Dockerfile entrypoint?

I'm trying to understand what this Docker entrypoint does . It seems to me that's a very common pattern when writing Dockerfiles, but my bash skills are limited and I have no idea of all the special bash symbols kung fu. Also, it's hard to google for "--", "$!" etc. What are these called in bash world? To summarize, what is the line bellow trying to do? if [ "${1#-}" != "$1" ]; then set -- haproxy "$@" fi