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 :tada: 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`). 

1 files changed, 8 insertions, 4 deletions

diff --git a/docker/README.md b/docker/README.md
index b65bcea636..a7d1e670fe 100644
--- a/docker/README.md
+++ b/docker/README.md
@@ -11,7 +11,7 @@ is not supported by this image.
 
 ## Volumes
 
-By default, the image expects a single volume, located at ``/data``, that will hold:
+By default, the image expects a single volume, located at `/data`, that will hold:
 
 * configuration files;
 * uploaded media and thumbnails;
@@ -19,11 +19,11 @@ By default, the image expects a single volume, located at ``/data``, that will h
 * the appservices configuration.
 
 You are free to use separate volumes depending on storage endpoints at your
-disposal. For instance, ``/data/media`` could be stored on a large but low
+disposal. For instance, `/data/media` could be stored on a large but low
 performance hdd storage while other files could be stored on high performance
 endpoints.
 
-In order to setup an application service, simply create an ``appservices``
+In order to setup an application service, simply create an `appservices`
 directory in the data volume and write the application service Yaml
 configuration file there. Multiple application services are supported.
 
@@ -56,6 +56,8 @@ The following environment variables are supported in `generate` mode:
 * `SYNAPSE_SERVER_NAME` (mandatory): the server public hostname.
 * `SYNAPSE_REPORT_STATS` (mandatory, `yes` or `no`): whether to enable
   anonymous statistics reporting.
+* `SYNAPSE_HTTP_PORT`: the port Synapse should listen on for http traffic.
+      Defaults to `8008`.
 * `SYNAPSE_CONFIG_DIR`: where additional config files (such as the log config
   and event signing key) will be stored. Defaults to `/data`.
 * `SYNAPSE_CONFIG_PATH`: path to the file to be generated. Defaults to
@@ -76,6 +78,8 @@ docker run -d --name synapse \
     matrixdotorg/synapse:latest
 ```
 
+(assuming 8008 is the port Synapse is configured to listen on for http traffic.)
+
 You can then check that it has started correctly with:
 
 ```
@@ -211,4 +215,4 @@ healthcheck:
 ## Using jemalloc
 
 Jemalloc is embedded in the image and will be used instead of the default allocator.
-You can read about jemalloc by reading the Synapse [README](../README.md)
+You can read about jemalloc by reading the Synapse [README](../README.md).