* Docker image: avoid changing user during `generate`

The intention was always that the config files get written as the initial user
(normally root) - only the data directory needs to be writable by Synapse. This
got changed in https://github.com/matrix-org/synapse/pull/5970, but that seems
to have been a mistake.

* Avoid changing user if no explicit UID is given

* changelog

relative links don't work when it's on dockerhub.

Signed-off-by: Dirk Klimpel <dirk@klimpel.org>

Signed-off-by: Chris Castle <chris@crc.io>

* Add healthcheck startup delay by 5secs and reduced interval check to 15s
to reduce waiting time for docker aware edge routers bringing an
instance online

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.

Co-authored-by: Will Hunt <willh@matrix.org>
Co-authored-by: Erik Johnston <erik@matrix.org>

`uploads_path` was a thing that was never used; most of it was removed in #6628
but a few vestiges remained.

Lots of people seem to get confused by this mode, and it's been deprecated
since Synapse 1.1.0. It's time for it to go.

* Add working build command for docker image

* Add changelog

Signed-off-by: Tobia De Koninck <LEDfan@users.noreply.github.com>

* 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.

Signed-off-by: Amir Zarrinkafsh <nightah@me.com>

... to help people escape env var hell

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.

Fixes #4830.

because this file is reproduced on dockerhub and relative paths don't work

Add missing backslash

Correct default POSTGRES_USER in Docker README

Signed-off-by: Jurrie Overgoor <1213142+Jurrie@users.noreply.github.com>

Signed-off-by: Jurrie Overgoor <1213142+Jurrie@users.noreply.github.com>

Also:

* Fix wrapping in docker readme
* Clean up some docs on the docker image
* a workaround for #4554

Link to contrib/docker

This addresses #3224

It's much easier to build the image via docker-compose instead of an
error-prone low-level docker call.

Signed-off-by: Benedikt Heine <bebe@bebehei.de>