Just learnt about effective UID/GID today. Vaguely recalled `setuid` but never understood its use.
Came across `USER` (in Dockerfile) and `user` (in Compose) and always thought it had to match the uid:gid of an existing user. Turns out, it doesn't - it is used to set (from here onwards) the uid:gid to be used in-place of that associated with the current user's ACTUAL (or "real") uid:gid
Very cool, no more creating user accounts inside of containers, not that it was a problem but this does simplify things, especially when using Docker images OTHER than those which I have created myself.