Use multi-stage build

[taliesins]

To minimize the size of the image we probably want to build haproxy in one container and then copy the build outputs into the next container. This way the build tools are not in the haproxy container (smaller/safer image).

https://docs.docker.com/build/building/multi-stage/ https://github.com/b4tman/docker-squid/blob/master/Dockerfile