diff --git a/changelog.d/13515.doc b/changelog.d/13515.doc
new file mode 100644
index 0000000000000000000000000000000000000000..a4d9d97dcb922c559dec1e2f6c275690f89e4709
--- /dev/null
+++ b/changelog.d/13515.doc
@@ -0,0 +1 @@
+Document that the `DOCKER_BUILDKIT=1` flag is needed to build the docker image.
\ No newline at end of file
diff --git a/docker/README.md b/docker/README.md
index 5b7de2fe382996e4f2a2b42bc855713d4e2794de..017f046c58acb2c62bdba88e18c12d81bcbaba4a 100644
--- a/docker/README.md
+++ b/docker/README.md
@@ -191,7 +191,7 @@ If you need to build the image from a Synapse checkout, use the following `docke
  build` command from the repo's root:
 
 ```
-docker build -t matrixdotorg/synapse -f docker/Dockerfile .
+DOCKER_BUILDKIT=1 docker build -t matrixdotorg/synapse -f docker/Dockerfile .
 ```
 
 You can choose to build a different docker image by changing the value of the `-f` flag to