Make (most) containers start as non-root

This makes all containers (except mautrix-telegram and
mautrix-whatsapp), start as a non-root user.

We do this, because we don't trust some of the images.
In any case, we'd rather not trust ALL images and avoid giving
`root` access at all. We can't be sure they would drop privileges
or what they might do before they do it.

Because Postfix doesn't support running as non-root,
it had to be replaced by an Exim mail server.

The matrix-nginx-proxy nginx container image is patched up
(by replacing its main configuration) so that it can work as non-root.
It seems like there's no other good image that we can use and that is up-to-date
(https://hub.docker.com/r/nginxinc/nginx-unprivileged is outdated).

Likewise for riot-web (https://hub.docker.com/r/bubuntux/riot-web/),
we patch it up ourselves when starting (replacing the main nginx
configuration).
Ideally, it would be fixed upstream so we can simplify.

roles/matrix-nginx-proxy/templates/systemd/matrix-nginx-proxy.service.j2

@@ -14,9 +14,11 @@ ExecStartPre=-/usr/bin/docker kill matrix-nginx-proxy
 ExecStartPre=-/usr/bin/docker rm matrix-nginx-proxy
 ExecStart=/usr/bin/docker run --rm --name matrix-nginx-proxy \
 			--log-driver=none \
+			--user={{ matrix_user_uid }}:{{ matrix_user_gid }} \
 			--network={{ matrix_docker_network }} \
-			-p 80:80 \
-			-p 443:443 \
+			-p 80:8080 \
+			-p 443:8443 \
+			-v {{ matrix_nginx_proxy_data_path }}/nginx.conf:/etc/nginx/nginx.conf:ro \
 			-v {{ matrix_nginx_proxy_confd_path }}:/etc/nginx/conf.d:ro \
 			-v {{ matrix_ssl_config_dir_path }}:{{ matrix_ssl_config_dir_path }}:ro \
 			-v {{ matrix_static_files_base_path }}:{{ matrix_static_files_base_path }}:ro \