Control when Docker Compose pulls a service image with pull_policy (always, never, missing, build, daily, weekly)

Documented steps

1. Add `pull_policy:` to a service with one of: always (pull every time), never (use local cache only, error if absent), missing (pull only if not cached — the default), build (rebuild the image), daily / weekly (re-pull if last pull is older than 24h / 7 days), or every_<duration> (e.g. every_12h or every_6h30m).
2. Example: `services: app: image: ghcr.io/acme/app:1.2.3 pull_policy: missing`.
3. Use `pull_policy: always` to force refreshing a mutable tag (e.g. a rolling deployment) instead of relying on an already-pulled layer.
4. Use `pull_policy: never` for reproducible offline or air-gapped runs where the image must already exist locally.
5. Docs: https://docs.docker.com/reference/compose-file/services/#pull_policy