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(https://hub.docker.com/r/nginxinc/nginx-unprivileged is outdated).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/defaults/main.yml

@@ -1,5 +1,8 @@
 matrix_nginx_proxy_enabled: true
 
+# We use an official nginx image, which we fix-up to run unprivileged.
+# An alternative would be an `nginxinc/nginx-unprivileged` image, but
+# those as more frequently out of date.
 matrix_nginx_proxy_docker_image: "nginx:1.15.8-alpine"
 
 matrix_nginx_proxy_data_path: "{{ matrix_base_data_path }}/nginx-proxy"