Feature Request

support fts-xapian as an alternative to fts-solr

Context

fts-xapian is an alternative full text search engine for dovecot that can be easier to tune for mailservers with limited RAM.

the tradeoff is that the indexes takes up much more disk space.

Is your Feature Request related to a Problem?

currently, dovecot-solr bindings are installed on docker-mailserver (which can be integrated with solr using a separate container ).

however, solr runs on the JVM, requires running an extra container, needs an additional volume and consumes a lot of memory (even when not indexing).

fts-xapian is an alternative that can be easily tuned for low memory environments. it does not need another container, it does not need an additional volume, and it can be easily tuned in the dovecot plugin itself.

Describe the Solution you'd like

for a fully integrated solution, dovecot-fts-xapian needs to be added to the Dockerfile. however, this package is only available in buster-backports, so that repo will need to be added to the apt list before installing.

What have you done so far?

currently, users can customize their existing docker-mailserver to use this plugin by adding a layer to the docker-mailserver container and then adding one dovecot plugin config file.

What are you going to contribute?

if desired, i could add the additional layer as described in this gist into the existing docker-mailserver container.

also, the existing FTS documentation could be appended with instructions on how to configure and turn on fts-xapian.