docker supports some volume mounts while building, it's part of the buildkit backend and is used for mounting secrets during builds. It is very poorly documented and hidden behind the buildkit buildx command. Check out the RUN --mount= section here: https://github.com/moby/buildkit/blob/master/frontend/docker...
It really is poorly documented. If you don't know it exists, it's difficult to find the document you've linked. I do know it exists and I still have a hard time finding it; it's barely mentioned (or not mentioned) in the regular Docker documentation, which is where most people will be looking.
Buildkit is not exclusive to docker buildx though. You can use this with regular docker build as long as you've set the DOCKER_BUILDKIT environment variable, as noted in the document. You can also forward this to docker-compose, though there's another COMPOSE_DOCKER_CLI_BUILD variable you must set for that.
That said, it looks like this extra frontend syntax is Docker-only, which means you shouldn't use it unless you're committed to the Docker tooling and ecosystem.
Yeah it's a royal mess right now unfortunately. It kind of seems like most of the work on docker's developer experience stopped in the last few years as the company went through turmoil and different ownership changes. Hopefully some day they sit down and really clarify what the 'golden path' is for a developer using docker today. Buildkit is really cool and can do a lot of nifty things once all these new features are enabled.
