chore: default Cloudron base to 5.0.0; parameterize template with __CLOUDRON_BASE__ and --base option

CloudronPackages/PackageTemplate/README.md

@@ -6,6 +6,7 @@ Replace placeholders in files with your app specifics:
 - `__APP_ID__` (e.g., com.example.myapp)
 - `__APP_TITLE__` (human name)
 - `__HTTP_PORT__` (default internal app port)
+- `__CLOUDRON_BASE__` (Cloudron base image tag, e.g., 5.0.0)
 
 Files
 - `CloudronManifest.json` – base manifest
@@ -21,4 +22,3 @@ Usage
    `scripts/new-package.sh MyApp --id com.example.myapp --title "My App" --port 3000`
 2. Adjust Dockerfile and start.sh to run your app.
 3. Build and test locally; then commit and push.
-