diff --git a/README.md b/README.md
index f985d46..35f8de0 100644
--- a/README.md
+++ b/README.md
@@ -92,6 +92,8 @@ Build the container (the container is named `backup-test`):
 ./build.sh
 ```
 
+The image needs BuildKit (default since Docker 23; on older Docker set `DOCKER_BUILDKIT=1`).
+
 Run the container:
 ```
 ./run.sh