Make optional RealESRGAN wheel prebuild opt-in

docs/wilkensxl-build.md

@@ -34,3 +34,15 @@ The loader does not remove all porting work, but it reduces it. Most local
 behavior should live behind addon manifests, so future upstream updates only
 need the loader compatibility checked plus any addons that touch changed
 internals.
+
+## Optional image extras
+
+The Docker image skips the optional Real-ESRGAN wheel prebuild by default. That
+keeps normal CI and Dockge updates from blocking on optional Gallery upscaling
+dependencies.
+
+To include the prebuilt Real-ESRGAN helper wheels in a custom image, build with:
+
+```text
+--build-arg PREBUILD_REALESRGAN_WHEELS=true
+```