Merge and override multiple Compose files in order with -f or COMPOSE_FILE

Documented steps

1. Pass multiple files in precedence order: `docker compose -f compose.yaml -f compose.prod.yaml up -d`. Later files override earlier ones.
2. Alternatively set COMPOSE_FILE to a colon-separated list of files, e.g. `export COMPOSE_FILE=compose.yaml:compose.prod.yaml`.
3. By convention Compose auto-loads compose.yaml plus compose.override.yaml (overrides) when found in the project directory.
4. Multi-value keys (ports, expose, dns, tmpfs) are concatenated; single-value keys (image, command, mem_limit) are replaced; environment/labels are merged by key with later taking precedence; volumes/devices are merged by container mount path.
5. Review the merged output with `docker compose config` to catch path or precedence mistakes.
6. Docs: https://docs.docker.com/compose/how-tos/multiple-compose-files/merge/