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`).
Context is in https://github.com/matrix-org/synapse/issues/9764#issuecomment-818615894.
I struggled to find a more official link for this. The problem occurs when using WSL1 instead of WSL2, which some Windows platforms (at least Server 2019) still don't have. Docker have updated their documentation to paint a much happier picture now given WSL2's support.
The last sentence here can probably be removed once WSL1 is no longer around... though that will likely not be for a very long time.
* Allow passing SYNAPSE_WORKER envvar
* changelog.d
* Document SYNAPSE_WORKER.
Attempting to imply that you don't need to change this default
unless you're in worker mode.
Also aware that there's a bigger problem of attempting to document
a complete working configuration of workers using docker, as we
currently only document to use `synctl` for worker mode, and synctl
doesn't work that way in docker.
This is mostly a documentation change, but also adds a default value for
SYNAPSE_CONFIG_PATH, so that running from the generated config is the default,
and will Just Work provided your config is in the right place.
