Add a script to update the debian changelog for non-Debian systems (#10778)

When releasing 1.42.0 with @Azrenbeth and talking with @clokep yesterday I realised doing the dch incantations related to releasing Synapse wasn't trivial on eg a macOS system, so this is a script to run in a Debian container to make things a bit easier.

changelog.d/10778.misc

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+Add a script to update the Debian changelog in a Docker container for systems that aren't Debian-based.

scripts-dev/docker_update_debian_changelog.sh

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+#!/bin/bash -e
+# Copyright 2021 The Matrix.org Foundation C.I.C.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# This script is meant to be used inside a Docker container to run the `dch` incantations
+# needed to release Synapse. This is useful on systems like macOS where such scripts are
+# not easily accessible.
+#
+# Running it (when if the current working directory is the root of the Synapse checkout):
+#   docker run --rm -v $PWD:/synapse ubuntu:latest /synapse/scripts-dev/docker_update_debian_changelog.sh VERSION
+#
+# The image can be replaced by any other Debian-based image (as long as the `devscripts`
+# package exists in the default repository).
+# `VERSION` is the version of Synapse being released without the leading "v" (e.g. 1.42.0).
+
+# Check if a version was provided.
+if [ "$#" -ne 1 ]; then
+  echo "Usage: update_debian_changelog.sh VERSION"
+  echo "VERSION is the version of Synapse being released in the form 1.42.0 (without the leading \"v\")"
+  exit 1
+fi
+
+# Check that apt-get is available on the system.
+if ! which apt-get > /dev/null 2>&1; then
+  echo "\"apt-get\" isn't available on this system. This script needs to be run in a Docker container using a Debian-based image."
+  exit 1
+fi
+
+# Check if devscripts is available in the default repos for this distro.
+# Update the apt package list cache.
+# We need to do this before we can search the apt cache or install devscripts.
+apt-get update || exit 1
+
+if ! apt-cache search devscripts | grep -E "^devscripts \-" > /dev/null; then
+  echo "The package \"devscripts\" needs to exist in the default repositories for this distribution."
+  exit 1
+fi
+
+# We set -x here rather than in the shebang so that if we need to exit early because no
+# version was provided, the message doesn't get drowned in useless output.
+set -x
+
+# Make the root of the Synapse checkout the current working directory.
+cd /synapse
+
+# Install devscripts (which provides dch). We need to make the Debian frontend
+# noninteractive because installing devscripts otherwise asks for the machine's location.
+DEBIAN_FRONTEND=noninteractive apt-get install -y devscripts
+
+# Update the Debian changelog.
+ver=${1}
+dch -M -v $(sed -Ee 's/(rc|a|b|c)/~\1/' <<<$ver) "New synapse release $ver."
+dch -M -r -D stable ""