Can someone school me on docker lifecycles?

Supposing I am building a docker image and sticking it on dockerhub, what part of the Dockerfile is run by me (builder) vs when someone runs the image?

Also multistage makes that even more complex, but I feel like I just don't get the lifecycle to begin with

ok, figured it out

each step generates an intermediate container hash

which get cached

so if you build an image, it will contain everything until the final CMD line