fix: open Jenkins in browser instead of VS Code preview

Change onAutoForward from 'openPreview' to 'openBrowser' to avoid
preview pane issues with Jenkins security headers.

The VS Code preview shows 'Please reopen the preview' due to Jenkins
X-Frame-Options headers. Opening in a regular browser tab works fine.

Updated README to clarify this behavior.

Author: gounthar

.devcontainer/README.md

@@ -35,6 +35,8 @@ After starting a tutorial with `docker compose --profile <name> up -d`:
 - Jenkins URL: `https://<codespace>-8080.<domain>` (shown in PORTS panel)
 - Default credentials: admin/admin
 
+**Important:** Open Jenkins in a regular browser tab, not the VS Code preview pane. The preview may show "Please reopen the preview" due to Jenkins security headers. Click the globe icon 🌐 in the PORTS panel or copy the URL to your browser.
+
 ## Troubleshooting
 
 **Port 8080 refuses connection:**

.devcontainer/devcontainer.json

@@ -20,7 +20,7 @@
   "portsAttributes": {
     "8080": {
       "label": "Jenkins Controller",
-      "onAutoForward": "openPreview",
+      "onAutoForward": "openBrowser",
       "protocol": "http",
       "visibility": "public"
     },