add devcontainer for GitHub Codespaces

Author: BattiestStone4

.devcontainer/devcontainer.json

@@ -0,0 +1,15 @@
+{
+  "name": "rCore Tutorial",
+  "build": {
+    "dockerfile": "../Dockerfile",
+    "context": ".."
+  },
+  "remoteUser": "root",
+  "customizations": {
+    "vscode": {
+      "extensions": [
+        "rust-lang.rust-analyzer"
+      ]
+    }
+  }
+}