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(https://hub.docker.com/r/nginxinc/nginx-unprivileged is outdated).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-mailer/templates/systemd/matrix-mailer.service.j2

@@ -9,8 +9,10 @@ ExecStartPre=-/usr/bin/docker kill matrix-mailer
 ExecStartPre=-/usr/bin/docker rm matrix-mailer
 ExecStart=/usr/bin/docker run --rm --name matrix-mailer \
 			--log-driver=none \
+			--user={{ matrix_mailer_container_user_uid }}:{{ matrix_mailer_container_user_gid }} \
 			--network={{ matrix_docker_network }} \
 			--env-file={{ matrix_mailer_base_path }}/env-mailer \
+			--hostname={{ hostname_matrix }} \
 			{{ matrix_mailer_docker_image }}
 ExecStop=-/usr/bin/docker kill matrix-mailer
 ExecStop=-/usr/bin/docker rm matrix-mailer