Mark Dockerfile as requiring BuildKit (#12541)

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

changelog.d/12541.docker

+Explicitly opt-in to using [BuildKit-specific features](https://github.com/moby/buildkit/blob/master/frontend/dockerfile/docs/syntax.md) in the Dockerfile. This fixes issues with building images in some GitLab CI environments.

docker/Dockerfile

+# syntax=docker/dockerfile:1
 # Dockerfile to build the matrixdotorg/synapse docker images.
 #
 # Note that it uses features which are only available in BuildKit - see