Call out buildkit is required when building test docker images (#13338)

Co-authored-by: David Robertson <davidr@element.io>

changelog.d/13338.doc

+Mention that BuildKit is needed when building Docker images for tests.

docker/Dockerfile-workers

+# syntax=docker/dockerfile:1
 # Inherit from the official Synapse docker image
 ARG SYNAPSE_VERSION=latest
 FROM matrixdotorg/synapse:$SYNAPSE_VERSION

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:

docker/complement/Dockerfile

+# 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.