Add support for running Complement against the local checkout

changelog.d/8317.feature

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+Support testing the local Synapse checkout against the [Complement homeserver test suite](https://github.com/matrix-org/complement/).

scripts-dev/complement.sh

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+#! /bin/bash -eu
+# This script is designed for developers who want to test their code
+# against Complement.
+#
+# It makes a Synapse image which represents the current checkout,
+# then downloads Complement and runs it with that image.
+
+cd "$(dirname $0)/.."
+
+# Build the base Synapse image from the local checkout
+docker build -t matrixdotorg/synapse:latest -f docker/Dockerfile .
+
+# Download Complement
+wget -N https://github.com/matrix-org/complement/archive/master.tar.gz
+tar -xzf master.tar.gz
+cd complement-master
+
+# Build the Synapse image from Complement, based on the above image we just built
+docker build -t complement-synapse -f dockerfiles/Synapse.Dockerfile ./dockerfiles
+
+# Run the tests on the resulting image!
+COMPLEMENT_BASE_IMAGE=complement-synapse go test -v -count=1 ./tests