[BUG] Strange env var behavior when using $var in var

[cgarnier]

Description

When i use 2 env files with a var composed of others vars, the resulted var is wrong.

Steps To Reproduce

.
├── docker-compose.yml
├── env1
└── env2

# docker-compose.yml
version: "3.7"
services:
  web:
    image: nginx
    environment:
      - MY_VAR
      - MY_COMP
      - MY_COMP2=BLABLA $MY_VAR

# env1
MY_VAR=1

# env2
MY_VAR=2
MY_COMP="BLABLA $MY_VAR"

Result of the docker compose --env-file env1 --env-file env2 config command:

services:
  web:
    environment:
      MY_COMP: BLABLA 1
      MY_COMP2: BLABLA 2
      MY_VAR: "2"
    image: nginx
    networks:
      default: null
networks:
  default:
    name: ef2_default

MY_COMP should be equal to BLABLA 2

Compose Version

Docker Compose version v2.21.0

Docker Environment

Client: Docker Engine - Community
 Version:    24.0.7
 Context:    default
 Debug Mode: false
 Plugins:
  buildx: Docker Buildx (Docker Inc.)
    Version:  v0.11.2
    Path:     /usr/libexec/docker/cli-plugins/docker-buildx
  compose: Docker Compose (Docker Inc.)
    Version:  v2.21.0
    Path:     /usr/libexec/docker/cli-plugins/docker-compose

Server:
 Containers: 5
  Running: 0
  Paused: 0
  Stopped: 5
 Images: 168
 Server Version: 24.0.7
 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: systemd
 Cgroup Version: 2
 Plugins:
  Volume: local
  Network: bridge host ipvlan macvlan null overlay
  Log: awslogs fluentd gcplogs gelf journald json-file local logentries splunk syslog
 Swarm: inactive
 Runtimes: io.containerd.runc.v2 runc
 Default Runtime: runc
 Init Binary: docker-init
 containerd version: 3dd1e886e55dd695541fdcd67420c2888645a495
 runc version: v1.1.10-0-g18a0cb0
 init version: de40ad0
 Security Options:
  apparmor
  seccomp
   Profile: builtin
  cgroupns
 Kernel Version: 6.2.0-39-generic
 Operating System: Ubuntu 22.04.3 LTS
 OSType: linux
 Architecture: x86_64
 CPUs: 32
 Total Memory: 31.25GiB
 Name: clement-desktop
 ID: 45a4fe3a-c174-4b96-b396-a5fb0219b8ff
 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]

related to https://github.com/compose-spec/compose-go/pull/403

the issue here is that we load env files sequentially, and while value set by user's environment has precedence, value from an earlier env file doesn't.

[github-actions[bot]]

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