Persist the Nuitka build cache

README.md

@@ -76,6 +76,16 @@ docker compose up -d
 docker compose ps
 ```
 
+The backend build exports its complete BuildKit cache to
+`.build-cache/backend`. This directory includes the intermediate Nuitka build
+layers. Keep it to reuse the compiled result after Docker removes its internal
+build cache. The directory can be large and is excluded from Git and the Docker
+build context.
+
+The Python builder image is pinned to a tested image digest. Update this digest
+deliberately when the compiler environment needs an update. A runtime base
+image update does not invalidate the Nuitka build layer.
+
 By default, the API is available at `http://localhost:17777/api`. Swagger UI is
 available at `http://localhost:17777/api/doc`.
 The default bind address is suitable for a reverse proxy on the Docker host.
@@ -108,7 +118,7 @@ Update the backend source code. Then, rebuild the containers:
 
 ```bash
 git -C upstream/HubBackend pull --ff-only
-docker compose build
+docker compose build backend
 docker compose up -d
 ```