Is your feature request related to a problem? Please describe.
Hi, docker-compose rocks, keep up the good work! 🙏
I had cases where the container should be running with the same user and group ID as the docker-compose process, e.g. so that files created inside the container will start out with the right owner without any need for initial root permissions (which is not cool for security) or chown'ing files et cetera.

As of today, inside docker-compose.yml a service definition can already ask for a specific user by means of syntax user: "<user_id>:<group_id>" but there are no predefined variables to effectively achieve something like user: "$(id -u):$(id -g)" from inside the YAML file with convenice. The list of Compose file and CLI variables does not document any variables like that.

Here's a few documented examples for other people trying to achieve the same, and they all need to resort to means outside the docker-compose.yml file: [1] [2] [3] [4] [5] [6].

Describe the solution you'd like
It would be great to have two more predefined variables:

• COMPOSE_USER_ID predefined with the effective user ID, as return by $(id -u)
• COMPOSE_GROUP_ID predefined with the effective group ID, as return by $(id -g)

With those two variables, a docker-compose.yml file could now use syntax user: "${COMPOSE_USER_ID?}:${COMPOSE_GROUP_ID?}" to conveniently have a container running as the same user as the one calling docker-compose. Wouldn't that be nice! 😃

Feature request #3849 seems to be rooted in a similar need, but my impression was that the door to arbitrary code execution has been kept shut for security reasons so far and adding two variables should work without questioning the current threat model.

Describe alternatives you've considered
You could define COMPOSE_USER_ID and COMPOSE_GROUP_ID outside of docker-compose.yml e.g.

COMPOSE_USER_ID="$(id -u)" COMPOSE_GROUP_ID="$(id -g)" docker-compose run ...

but it's not very convenient, does not work out of the box, and it's not standardized.

Additional context

# cat docker-compose.yml 
version: "3"

services:
  debian:
    image: debian:sid
    user: "${COMPOSE_USER_ID?}:${COMPOSE_GROUP_ID?}"

# COMPOSE_USER_ID="$(id -u)" COMPOSE_GROUP_ID="$(id -g)" docker-compose run debian sh -c id  # unpatched
Creating tmpyvecgpc8mx_debian_run ... done
uid=1000 gid=1000 groups=1000

# docker-compose run debian sh -c id  # patched future release, same effect
Creating tmpyvecgpc8mx_debian_run ... done
uid=1000 gid=1000 groups=1000