Add a dockerfile for running a set of Synapse worker processes (#9162)

This PR adds a Dockerfile and some supporting files to the `docker/` directory. The Dockerfile's intention is to spin up a container with:

* A Synapse main process.
* Any desired worker processes, defined by a `SYNAPSE_WORKERS` environment variable supplied at runtime.
* A redis for worker communication.
* A nginx for routing traffic.
* A supervisord to start all worker processes and monitor them if any go down.

Note that **this is not currently intended to be used in production**. If you'd like to use Synapse workers with Docker, instead make use of the official image, with one worker per container. The purpose of this dockerfile is currently to allow testing Synapse in worker mode with the [Complement](https://github.com/matrix-org/complement/) test suite.

`configure_workers_and_start.py` is where most of the magic happens in this PR. It reads from environment variables (documented in the file) and creates all necessary config files for the processes. It is the entrypoint of the Dockerfile, and thus is run any time the docker container is spun up, recreating all config files in case you want to use a different set of workers. One can specify which workers they'd like to use by setting the `SYNAPSE_WORKERS` environment variable (as a comma-separated list of arbitrary worker names) or by setting it to `*` for all worker processes. We will be using the latter in CI.

Huge thanks to @MatMaul for helping get this all working 🎉 This PR is paired with its equivalent on the Complement side: https://github.com/matrix-org/complement/pull/62.

Note, for the purpose of testing this PR before it's merged: You'll need to (re)build the base Synapse docker image for everything to work (`matrixdotorg/synapse:latest`). Then build the worker-based docker image on top (`matrixdotorg/synapse:workers`).

docker/conf-workers/nginx.conf.j2

@@ -0,0 +1,27 @@
+# This file contains the base config for the reverse proxy, as part of ../Dockerfile-workers.
+# configure_workers_and_start.py uses and amends to this file depending on the workers
+# that have been selected.
+
+{{ upstream_directives }}
+
+server {
+    # Listen on an unoccupied port number
+    listen 8008;
+    listen [::]:8008;
+
+    server_name localhost;
+
+    # Nginx by default only allows file uploads up to 1M in size
+    # Increase client_max_body_size to match max_upload_size defined in homeserver.yaml
+    client_max_body_size 100M;
+
+{{ worker_locations }}
+
+    # Send all other traffic to the main process
+    location ~* ^(\\/_matrix|\\/_synapse) {
+        proxy_pass http://localhost:8080;
+        proxy_set_header X-Forwarded-For $remote_addr;
+        proxy_set_header X-Forwarded-Proto $scheme;
+        proxy_set_header Host $host;
+    }
+}

docker/conf-workers/shared.yaml.j2

@@ -0,0 +1,9 @@
+# This file contains the base for the shared homeserver config file between Synapse workers,
+# as part of ./Dockerfile-workers.
+# configure_workers_and_start.py uses and amends to this file depending on the workers
+# that have been selected.
+
+redis:
+    enabled: true
+
+{{ shared_worker_config }}

docker/conf-workers/supervisord.conf.j2

@@ -0,0 +1,41 @@
+# This file contains the base config for supervisord, as part of ../Dockerfile-workers.
+# configure_workers_and_start.py uses and amends to this file depending on the workers
+# that have been selected.
+[supervisord]
+nodaemon=true
+user=root
+
+[program:nginx]
+command=/usr/sbin/nginx -g "daemon off;"
+priority=500
+stdout_logfile=/dev/stdout
+stdout_logfile_maxbytes=0
+stderr_logfile=/dev/stderr
+stderr_logfile_maxbytes=0
+username=www-data
+autorestart=true
+
+[program:redis]
+command=/usr/bin/redis-server /etc/redis/redis.conf --daemonize no
+priority=1
+stdout_logfile=/dev/stdout
+stdout_logfile_maxbytes=0
+stderr_logfile=/dev/stderr
+stderr_logfile_maxbytes=0
+username=redis
+autorestart=true
+
+[program:synapse_main]
+command=/usr/local/bin/python -m synapse.app.homeserver --config-path="{{ main_config_path }}" --config-path=/conf/workers/shared.yaml
+priority=10
+# Log startup failures to supervisord's stdout/err
+# Regular synapse logs will still go in the configured data directory
+stdout_logfile=/dev/stdout
+stdout_logfile_maxbytes=0
+stderr_logfile=/dev/stderr
+stderr_logfile_maxbytes=0
+autorestart=unexpected
+exitcodes=0
+
+# Additional process blocks
+{{ worker_config }}

docker/conf-workers/worker.yaml.j2

@@ -0,0 +1,26 @@
+# This is a configuration template for a single worker instance, and is
+# used by Dockerfile-workers.
+# Values will be change depending on whichever workers are selected when
+# running that image.
+
+worker_app: "{{ app }}"
+worker_name: "{{ name }}"
+
+# The replication listener on the main synapse process.
+worker_replication_host: 127.0.0.1
+worker_replication_http_port: 9093
+
+worker_listeners:
+  - type: http
+    port: {{ port }}
+{% if listener_resources %}
+    resources:
+      - names:
+{%- for resource in listener_resources %}
+        - {{ resource }}
+{%- endfor %}
+{% endif %}
+
+worker_log_config: {{ worker_log_config_filepath }}
+
+{{ worker_extra_conf }}