chore(n): honor docker-compose.override.yml for the main dev stack

[adekbadek]

All Submissions:

• Have you followed the Newspack Contributing guidelines?
• Does your code follow the WordPress coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

n start invokes Docker Compose with an explicit -f docker-compose.yml, which disables Compose's automatic loading of docker-compose.override.yml. As a result, a developer's root override (e.g. extra bind mounts) was silently ignored.

This makes n start append -f docker-compose.override.yml to the Compose invocation only when that file exists, so its settings merge over the base stack. When the file is absent, the invocation is unchanged, so the default behavior is fully preserved.

The override is scoped to the main dev stack (newspack_dev). Isolated environments (n env) intentionally do not load it; they already layer their own generated docker-compose.env-*.yml overrides, and folding a global override into them could clobber env-specific config via file ordering. The override file is gitignored (it's a per-developer customization), and the mechanism is documented in AGENTS.md.

Reported by Wil (Slack thread).

How to test the changes in this Pull Request:

1. Check out this branch and cp .env into place if needed (or run from your normal workspace with .env present).
2. Create a throwaway docker-compose.override.yml at the workspace root that adds a bind mount to the wordpress service:

   services:
     wordpress:
       volumes:
         - /tmp/np-override-probe:/var/www/override-probe

3. Run docker compose -f docker-compose.yml -f docker-compose.override.yml config (what n start now invokes) and confirm the merged output contains the /var/www/override-probe mount. Equivalently, run n start and verify the mount is live: docker exec newspack_dev ls -la /var/www/override-probe.
4. Delete docker-compose.override.yml, run n start again, and confirm the container comes up exactly as before (no probe mount) – default behavior is preserved when no override is present.
5. Confirm isolated environments are unaffected: n env up <name> still works and does not pick up the root override.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully run tests with your changes locally?

[Copilot]

Pull request overview

Makes n start honor a developer-local docker-compose.override.yml for the main dev stack, restoring Compose's standard override behavior that was disabled by the explicit -f flag.

Changes:

• Append -f docker-compose.override.yml to the docker compose up invocation when the file exists.
• Gitignore the override file so it stays per-developer.
• Document the override mechanism in AGENTS.md.

Reviewed changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated no comments.

File	Description
n	Conditionally adds the override file to the compose invocation in start().
AGENTS.md	Documents how to customize the main dev stack via docker-compose.override.yml.
.gitignore	Ignores the new docker-compose.override.yml file.