Add support for running Complement against the local checkout (#8317)

This PR adds a script that:

* Builds the local Synapse checkout using our existing `docker/Dockerfile` image.
* Downloads [Complement](https://github.com/matrix-org/complement/)'s source code.
* Builds the [Synapse.Dockerfile](https://github.com/matrix-org/complement/blob/master/dockerfiles/Synapse.Dockerfile) using the above dockerfile as a base.
* Builds and runs Complement against it.

This set up differs slightly from [that of the dendrite repo](https://github.com/matrix-org/dendrite/blob/master/build/scripts/complement.sh) (`complement.sh`, `Complement.Dockerfile`), which instead stores a separate, but slightly modified, dockerfile in Dendrite's repo rather than running the one stored in Complement's repo. That synapse equivalent to that dockerfile (`Synapse.Dockerfile`) in Complement's repo is just based on top of `matrixdotorg/synapse:latest`, which we opt to build here locally.

Thus copying over the files from Complement's repo wouldn't change any functionality, and would result in two instances of the same files. So just using the dockerfile in Complement's repo was decided upon instead.

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