diff --git a/docker/Dockerfile b/docker/Dockerfile
index 6f87702cc8..575f9be861 100644
--- a/docker/Dockerfile
+++ b/docker/Dockerfile
@@ -1,3 +1,4 @@
+# syntax=docker/dockerfile:experimental
 # Dockerfile to build the matrixdotorg/synapse docker images.
 #
 # Note that it uses features which are only available in BuildKit - see