[BUG] `--index` for `docker-compose exec` command does not choose right container

[gburzynski-10clouds]

Description

I have 2 containers created from the same service server. One with backend, second with tests. I want to exec into backend container.

e99c12cd8eeb   d1cc9cdbcd1b   "./run_tests.sh"   28 minutes ago   Up 28 minutes                                                     server-run-68082c734962
e24541534a85   d1cc9cdbcd1b   "python manage.py ru…"   29 minutes ago   Up 29 minutes   0.0.0.0:8001->8001/tcp, :::8001->8001/tcp         server

Doesn't matter if I run:
docker-compose --file docker-compose.yml exec -t --index 0 server bash
or docker-compose --file docker-compose.yml exec -t --index 1 server bash
I end up inside test container (can be checked by looking into running processes ps aux.)

Workaround: docker-compose exec execs always into container that was created last.

Steps To Reproduce

1. Define server service in docker-compose.yml
2. Run one instance docker-compose --file docker-compose.yml run --rm --name server server python
3. Run second instance docker-compose --file docker-compose.yml run --rm server ./run_tests.sh
4. See the docker ps -a output to get the index of container
5. Notice that passed index does not change to which container we exec into docker-compose --file docker-compose.yml exec -t --index 1 server bash

Compose Version

2.29.7

Docker Environment

docker info
Client: Docker Engine - Community
Version: 27.3.1
Context: colima-redacted
Debug Mode: false

Server:
Containers: 4
Running: 4
Paused: 0
Stopped: 0
Images: 4
Server Version: 27.1.1
Storage Driver: overlay2
Backing Filesystem: extfs
Supports d_type: true
Using metacopy: false
Native Overlay Diff: true
userxattr: false
Logging Driver: json-file
Cgroup Driver: cgroupfs
Cgroup Version: 2
Plugins:
Volume: local
Network: bridge host ipvlan macvlan null overlay
Log: awslogs fluentd gcplogs gelf journald json-file local splunk syslog
Swarm: inactive
Runtimes: io.containerd.runc.v2 runc
Default Runtime: runc
Init Binary: docker-init
containerd version: 472731909fa34bd7bc9c087e4c27943f9835f111
runc version: v1.1.13-0-g58aa920
init version: de40ad0
Security Options:
apparmor
seccomp
Profile: builtin
cgroupns
Kernel Version: 6.8.0-41-generic
Operating System: Ubuntu 24.04.1 LTS
OSType: linux
Architecture: aarch64
CPUs: 8
Total Memory: 9.69GiB
Name: colima-redacted
ID: af1bee08-e490-44cd-a00a-c182c012f6f4
Docker Root Dir: /var/lib/docker
Debug Mode: false
Experimental: false
Insecure Registries:
127.0.0.0/8
Live Restore Enabled: false

Anything else?

No response

[ndeloof]

--index applies to containers created by up command. When you target containers created by run it has no effect, as such container as not indexed. Flag should be rejected to avoid confusion, so let's keep this tagged as a bug