.devcontainer: tweaks (#373427)

.devcontainer/devcontainer.json

···
   "image": "mcr.microsoft.com/devcontainers/universal:2-linux",
   "features": {
     "ghcr.io/devcontainers/features/nix:1": {
-      "multiUser": false
+      // fails in the devcontainer sandbox, enable sandbox via config instead
+      "multiUser": false,
+      "packages": "nixd,nixfmt-unstable",
+      "extraNixConfig": "experimental-features = nix-command flakes,sandbox = true"
+    }
+  },
+  "customizations": {
+    "vscode": {
+      "extensions": [
+        "jnoortheen.nix-ide"
+      ],
+      "settings": {
+        "nix.formatterPath": "nixfmt-rfc-style",
+        "nix.enableLanguageServer": true,
+        "nix.serverPath": "nixd"
+      }
     }
   },
   "remoteEnv": {