test(e2e): add docker-based end-to-end coverage for --backups-root and --force-keep

- Run E2E suite via unittest discovery inside the container
- Add E2E test for --id mode with real filesystem + fake dirval
- Add E2E test for --all + --force-keep to ensure latest backups are skipped

https://chatgpt.com/share/6954d89e-bf08-800f-be4a-5d237d190ddd

README.md

@@ -100,6 +100,7 @@ Scans:
 | `--workers <n>`      | Parallel workers (default: CPU count, min 2)                       |
 | `--timeout <sec>`    | Per-directory validation timeout (float supported, default: 300.0) |
 | `--yes`              | Non-interactive mode: delete failures automatically                |
+| `--force-keep <n>`   | In `--all` mode: skip the last *n* backup folders (default: 0)      |
 
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