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>