[docs] update dev guide runpod image to py3.12 (#602)

Author: SolitaryThinker

docs/source/contributing/developer_env/runpod.md

@@ -8,12 +8,14 @@ You can easily use the FastVideo Docker image as a custom container on [RunPod](
 
 Choose a GPU that supports CUDA 12.4
 
+Pick 1 or 2 L40S GPU(s)
+
 ![RunPod CUDA selection](../../_static/images/runpod_cuda.png)
 
 When creating your pod template, use this image:
 
 ```
-ghcr.io/hao-ai-lab/fastvideo/fastvideo-dev:latest
+ghcr.io/hao-ai-lab/fastvideo/fastvideo-dev:py3.12-latest
 ```
 
 Paste Container Start Command to support SSH ([RunPod Docs](https://docs.runpod.io/pods/configuration/use-ssh)):