[rspamd] Allow to easily use custom rspamd lua plugins

Since rspamd 1.9.2 we'll be able to load custom modules from plugins.d
directory.

This allow to add and configure plugins easily from the
data/conf/rspamd/plugins.d

Also loading config for custom plugins need rspamd.conf.local or
optionally rspamd.conf.override.

I added support for this in the docker-compose.yml

Idea came while i was writing a custom plugin for Cyren antispam
gateway, which can be found here: https://github.com/sriccio/rspamd-plugins