fix: include env interpolation in compose includes

[MaheshThakur9152]

Problem
When a top-level include supplies variables via env_file, those variables were not available later when parsing env_file referenced by services in the included compose file. Repro:

# compose.yml
include:
  - path: subproj/subcompose.yml
    env_file:
      - values.env

# subproj/subcompose.yml
services:
  app:
    env_file: subvalues.env
    image: helloworld

# subproj/subvalues.env
MYVAR=${VAR?}

# values.env
VAR=1

docker compose config failed with: required variable VAR is missing a value.

Fix
Merge variables read from an include's env_file into the parent environment so they are available when service-level env_file files are parsed later during project resolution. This is a small, non-invasive change in include.go.

Files changed

• include.go — merge include env_file vars into parent environment 🔁
• include_test.go — added TestIncludeEnvFileInterpolation (regression test) ✅

Tests

• Added unit test TestIncludeEnvFileInterpolation in loader package.
• Please verify locally or rely on CI:
  • go test ./loader -run TestIncludeEnvFileInterpolation -v
  • go test mahesh.

Compatibility & scope

• No public API changes. Behavior is more consistent with expectation that include-provided variables are usable by included files’ env_files.
• Minimal change consistent with existing loader architecture.

Suggested commit message

loader: merge include env_file vars so subproject env_file interpolation works

- Merge env vars read from include.env_file into parent env so service env_files
  inside included compose files can interpolate those variables.
- Add TestIncludeEnvFileInterpolation to cover regression.

Checklist for reviewers

• Confirm unit tests pass in CI (go test mahesh.)
• Validate tests cover the reported repro & edge-case precedence
• Confirm change is minimally invasive and well documented in PR/changelog

Related issue

• Possibly related to [BUG] Variables not interpolated in env_file in included subproject docker/compose#13540 (original report)

[ndeloof]

parent MUST not get those variables, they only SHOULD apply to included compose file. parent project to declare the same variable SHOULD not get value resolved