Update dev container config to support live code sync and improve docker setup guide (#6018)

Signed-off-by: Lifu Huang <lifu.hlf@gmail.com>

.devcontainer/devcontainer.json

@@ -20,5 +20,11 @@
     "runArgs": [
         "--gpus",
         "all"
-    ]
+    ],
+    // The two lines below ensures that your local changes in the sglang
+    // repo is automatically synced to the sglang pip package installed
+    // in the dev docker container. You can remove / comment out these
+    // two lines if you prefer to sync code changes manually.
+    "workspaceMount": "source=${localWorkspaceFolder},target=/sgl-workspace/sglang,type=bind",
+    "workspaceFolder": "/sgl-workspace/sglang"
 }

docs/developer/development_guide_using_docker.md

 # Development Guide Using Docker
 
-## Setup VSCode
+## Setup VSCode on a Remote Host
+(Optional - you can skip this step if you plan to run sglang dev container locally)
 
-Download `code` from `Https://code.visualstudio.com/docs/?dv=linux64cli`
+1. In the remote host, download `code` from [Https://code.visualstudio.com/docs/?dv=linux64cli](https://code.visualstudio.com/download) and run `code tunnel` in a shell.
 
+Example
 ```bash
 wget https://vscode.download.prss.microsoft.com/dbazure/download/stable/fabdb6a30b49f79a7aba0f2ad9df9b399473380f/vscode_cli_alpine_x64_cli.tar.gz
 tar xf vscode_cli_alpine_x64_cli.tar.gz
@@ -12,8 +14,31 @@ tar xf vscode_cli_alpine_x64_cli.tar.gz
 ./code tunnel
 ```
 
+2. In your local machine, press F1 in VSCode and choose "Remote Tunnels: Connect to Tunnel".
+
 ## Setup Docker Container
 
+### Option 1. Use the default dev container automatically from VSCode
+There is a `.devcontainer` folder in the sglang repository root folder to allow VSCode to automatically start up within dev container. You can read more about this VSCode extension in VSCode official document [Developing inside a Container](https://code.visualstudio.com/docs/devcontainers/containers).
+![image](https://github.com/user-attachments/assets/6a245da8-2d4d-4ea8-8db1-5a05b3a66f6d)
+(*Figure 1: Diagram from VSCode official documentation [Developing inside a Container](https://code.visualstudio.com/docs/devcontainers/containers).*)
+
+To enable this, you only need to:
+1. Start Visual Studio Code and install [VSCode dev container extension](https://marketplace.visualstudio.com/items?itemName=ms-vscode-remote.remote-containers).
+2. Press F1, type and choose "Dev Container: Open Folder in Container.
+3. Input the `sglang` local repo path in your machine and press enter.
+
+The first time you open it in dev container might take longer due to docker pull and build. Once it's successful, you should set on your status bar at the bottom left displaying that you are in a dev container:
+
+![image](https://github.com/user-attachments/assets/650bba0b-c023-455f-91f9-ab357340106b)
+
+Now when you run `sglang.launch_server` in the VSCode terminal or start debugging using F5, sglang server will be started in the dev container with all your local changes applied automatically:
+
+![image](https://github.com/user-attachments/assets/748c85ba-7f8c-465e-8599-2bf7a8dde895)
+
+
+### Option 2. Start up containers manually (advanced)
+
 The following startup command is an example for internal development by the SGLang team. You can **modify or add directory mappings as needed**, especially for model weight downloads, to prevent repeated downloads by different Docker containers.
 
 ❗️ **Note on RDMA**
@@ -21,20 +46,51 @@ The following startup command is an example for internal development by the SGLa
     1. `--network host` and `--privileged` are required by RDMA. If you don't need RDMA, you can remove them but keeping them there does not harm. Thus, we enable these two flags by default in the commands below.
     2. You may need to set `NCCL_IB_GID_INDEX` if you are using RoCE, for example: `export NCCL_IB_GID_INDEX=3`.
 
-### H100
-
 ```bash
 # Change the name to yours
+docker run -itd --shm-size 32g --gpus all -v <volumes-to-mount> --ipc=host --network=host --privileged --name sglang_dev lmsysorg/sglang:dev /bin/zsh
+docker exec -it sglang_dev /bin/zsh
+```
+Some useful volumes to mount are:
+1. **Huggingface model cache**: mounting model cache can avoid re-download everytime docker restarts. Default location on Linux is `~/.cache/huggingface/`.
+2. **SGLang repository**: code changes in the SGLang local repository will be automatically synced to the .devcontainer.
+
+Example 1: Monting local cache folder `/opt/dlami/nvme/.cache` but not the SGLang repo. Use this when you prefer to manually transfer local code changes to the devcontainer.
+```bash
 docker run -itd --shm-size 32g --gpus all -v /opt/dlami/nvme/.cache:/root/.cache --ipc=host --network=host --privileged --name sglang_zhyncs lmsysorg/sglang:dev /bin/zsh
 docker exec -it sglang_zhyncs /bin/zsh
 ```
-
-### H200
-
+Example 2: Mounting both HuggingFace cache and local SGLang repo. Local code changes are automatically synced to the devcontainer as the SGLang is installed in editable mode in the dev image.
 ```bash
-docker run -itd --shm-size 32g --gpus all -v /mnt/co-research/shared-models:/root/.cache/huggingface --ipc=host --network=host --privileged --name sglang_zhyncs lmsysorg/sglang:dev /bin/zsh
+docker run -itd --shm-size 32g --gpus all -v $HOME/.cache/huggingface/:/root/.cache/huggingface -v $HOME/src/sglang:/sgl-workspace/sglang --ipc=host --network=host --privileged --name sglang_zhyncs lmsysorg/sglang:dev /bin/zsh
 docker exec -it sglang_zhyncs /bin/zsh
 ```
+## Debug SGLang with VSCode Debugger
+1. (Create if not exist) open `launch.json` in VSCode.
+2. Add the following config and save. Please note that you can edit the script as needed to apply different parameters or debug a different program (e.g. benchmark script).
+     ```JSON
+       {
+          "version": "0.2.0",
+          "configurations": [
+              {
+                  "name": "Python Debugger: launch_server",
+                  "type": "debugpy",
+                  "request": "launch",
+                  "module": "sglang.launch_server",
+                  "console": "integratedTerminal",
+                  "args": [
+                      "--model-path", "meta-llama/Llama-3.2-1B",
+                      "--host", "0.0.0.0",
+                      "--port", "30000",
+                      "--trust-remote-code",
+                  ],
+                  "justMyCode": false
+              }
+          ]
+      }
+    ```
+
+3. Press "F5" to start. VSCode debugger will ensure that the program will pause at the breakpoints even if the program is running at remote SSH/Tunnel host + dev container.
 
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