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>