1320: Add xapian full-text-search plugin to dovecot r=mergify[bot] a=Nebukadneza
## What type of PR?
Enhancement
## What does this PR do?
Currently we are not able to offer our users a FTS experience after the
demise of lucene due to unfixed coredumps with musl/alpine.
We now add lucene, the only remaining maintained small/lean FTS plugin
for dovecot. It is quite simple to add to our stack: A two-stage docker
build is used to compile the fts plugin in the first stage, and copy
over only the resulting plugin-artifact to the second stage, which is
our usual dovecot container. Configuration is also minimal.
There was a upstream issue where bodies were not able to be searched for subwords, but fortunately it was fixed quite quickly. We currently need to wait for a new release to use a stable tag in our `Dockerfile`.
### Related issue(s)
- https://github.com/Mailu/Mailu/pull/1176
- https://github.com/Mailu/Mailu/pull/1297
- https://github.com/Mailu/Mailu/issues/751
- **Upstream-issues which is the cause for the `TODO` in the `Dockerfile`**: https://github.com/grosjo/fts-xapian/issues/32
## Prerequistes
- [ ] Wait for upstream to prepare new release after https://github.com/grosjo/fts-xapian/issues/32 — so that we can use a stable tag in our `Dockerfile`
- [ ] In case of feature or enhancement: documentation updated accordingly
- [ ] Unless it's docs or a minor change: add [changelog](https://mailu.io/master/contributors/guide.html#changelog) entry file.
Co-authored-by: Dario Ernst <dario@kanojo.de>
Co-authored-by: Dario Ernst <dario.ernst@rommelag.com>
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