chore: add devcontainer Dockerfile, devcontainer.json, and env (#997)

* chore: add devcontainer Dockerfile, devcontainer.json, and env

* style: remove unnecessary comment on ENABLE_CACHE setting

* docs: add internal devcontainer README with setup instructions

* Update README in .devcontainer based on PR #997

---------

Co-authored-by: jingyuanlm <842442862@qq.com>

.devcontainer/README.md

@@ -0,0 +1,30 @@
+# Introduction
+
+!!!!!This dev container is not for public development!!!!!!
+!!!!!Please don't use it if you are just a public open-source user.!!!!!!
+
+# Steps to run the dev container (for internal use only)
+
+Prerequisites(this is the reason why this dev container is not for public use):
+
+- Make sure you have the `rdagentappregistry.azurecr.io/rd-agent-mle:20250623` image locally & DevContainer is installed in your IDE
+- The kaggle dataset is located at `/home/shared/RD-Agent/kaggle`
+
+1. Open the project and select "Open In DevContainer"
+2. Set up your Kaggle Key (do not share this; other internal URLs are hardcoded in the config files)
+
+```bash
+export KAGGLE_USERNAME=
+export KAGGLE_KEY=
+```
+
+3. Run: python rdagent/app/data_science/loop.py --competition nomad2018-predict-transparent-conductors
+
+
+# Additional Notes
+- Please install and use this Dev Container in VS Code.
+- You **must open VS Code remotely and enter the `RD-Agent` directory before running the DevContainer configuration (`.devcontainer/devcontainer.json`)**. Otherwise, the workspace and path mappings will not work as expected.
+- To open the DevContainer correctly in VS Code:
+  1. Remotely connect to the machine and open the `RD-Agent` folder in VS Code.
+  2. Press `Ctrl+Shift+P` (or `Cmd+Shift+P` on Mac), type and select **"Dev Containers: Reopen in Container"**.
+