Replace torch image by vllm image in Docker

> Note that if we want to use `vllm` in the images for e.g. GRPO, a better approach is to use the official vllm image like this:
>
> ```suggestion
> FROM vllm/vllm-openai:v0.10.2
> ```

_Originally posted by @lewtun in [#4522](https://github.com/huggingface/trl/pull/4522/files#r2523945092)_