[Bug] Kata sandbox startup fails for non-root image users

## Description

When a sandbox image specifies a non-root default user, such as `USER admin`,
the sandbox startup script fails when the Kata runtime is enabled.

`rock/rocklet/local_files/docker_run.sh` tries to create the Docker data-root directory:

```bash
mkdir -p /var/lib/docker
```

The non-root user does not have permission to create this directory. Because the
script enables `set -o errexit`, the permission error terminates the startup
script before Rocklet can start.

## Steps to reproduce

1. Prepare a sandbox image with a non-root default user:

```dockerfile
FROM python:3.11
RUN useradd -m admin
USER admin
```

2. Start a sandbox with this image and enable `use_kata_runtime`.
3. Observe the sandbox startup logs.

## Actual behavior

The startup script exits with a permission error:

```text
Kata runtime detected, setting up DinD disk...
mkdir: cannot create directory '/var/lib/docker': Permission denied
```

Rocklet is not started, and the sandbox fails its startup health check.

## Expected behavior

Before creating the Docker data-root directory, the startup script should check
whether the current user has permission to create it.

If the directory cannot be created:

- print a clear warning;
- skip Kata DinD disk initialization;
- continue starting Rocklet.

Writable custom Docker `data-root` paths, including multi-level paths, should
continue to work.

## Environment

- ROCK: `master` at `1faec82d3`
- Runtime: Kata
- Sandbox image default user: non-root

Start with rock/rocklet/local_files/docker_run.sh and reproduce the Kata startup using an image whose default user is non-root. Check the permission failure around /var/lib/docker and custom multi-level data-root paths; done means an inaccessible path warns and skips DinD disk initialization while Rocklet starts, and writable paths still work.

Written by the indexing model from the issue text.