diff --git a/changelog.d/13338.doc b/changelog.d/13338.doc
new file mode 100644
index 000000000..7acf6d3f3
--- /dev/null
+++ b/changelog.d/13338.doc
@@ -0,0 +1 @@
+Mention that BuildKit is needed when building Docker images for tests.
diff --git a/docker/Dockerfile-workers b/docker/Dockerfile-workers
index 0f1570cfb..84f836ff7 100644
--- a/docker/Dockerfile-workers
+++ b/docker/Dockerfile-workers
@@ -1,3 +1,4 @@
+# syntax=docker/dockerfile:1
 # Inherit from the official Synapse docker image
 ARG SYNAPSE_VERSION=latest
 FROM matrixdotorg/synapse:$SYNAPSE_VERSION
diff --git a/docker/README-testing.md b/docker/README-testing.md
index 1f0423f09..21b99963d 100644
--- a/docker/README-testing.md
+++ b/docker/README-testing.md
@@ -22,6 +22,10 @@ Consult the [contributing guide][guideComplementSh] for instructions on how to u
 Under some circumstances, you may wish to build the images manually.
 The instructions below will lead you to doing that.
 
+Note that these images can only be built using [BuildKit](https://docs.docker.com/develop/develop-images/build_enhancements/),
+therefore BuildKit needs to be enabled when calling `docker build`. This can be done by
+setting `DOCKER_BUILDKIT=1` in your environment.
+
 Start by building the base Synapse docker image. If you wish to run tests with the latest
 release of Synapse, instead of your current checkout, you can skip this step. From the
 root of the repository:
diff --git a/docker/complement/Dockerfile b/docker/complement/Dockerfile
index c5e7984a2..3cfff19f9 100644
--- a/docker/complement/Dockerfile
+++ b/docker/complement/Dockerfile
@@ -1,3 +1,4 @@
+# syntax=docker/dockerfile:1
 # This dockerfile builds on top of 'docker/Dockerfile-workers' in matrix-org/synapse
 # by including a built-in postgres instance, as well as setting up the homeserver so
 # that it is ready for testing via Complement.