Reading the Docker Socket Proxy page, it doesn’t become clear to me how one could access it from another container? Seems to be missing from the page!
its… right in the application setup section of what you linked…
Most containers that would normally connect to a mounted docker.sock can have their endpoint overridden using the DOCKER_HOST environment variable if they do not offer the option in their configuration; it should typically be pointed to tcp://socket-proxy:2375.