Add xapian full-text-search plugin to dovecot

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.

core/dovecot/Dockerfile

@@ -1,4 +1,15 @@
 ARG DISTRO=alpine:3.10
+FROM $DISTRO
+WORKDIR /tmp
+RUN apk add git build-base automake autoconf libtool dovecot-dev xapian-core-dev icu-dev
+RUN git clone https://github.com/grosjo/fts-xapian.git \
+  && cd fts-xapian \
+  # && git checkout 1.2.7 \ # TODO: when upstream releases 1.2.7, use that
+  && autoreconf -vi \
+  && PANDOC=false ./configure --with-dovecot=/usr/lib/dovecot \
+  && make \
+  && make install
+
 FROM $DISTRO
 # python3 shared with most images
 RUN apk add --no-cache \
@@ -13,9 +24,11 @@ RUN pip3 install "podop>0.2.5"
 
 # Image specific layers under this line
 RUN apk add --no-cache \
-  dovecot dovecot-lmtpd dovecot-pop3d dovecot-submissiond dovecot-pigeonhole-plugin rspamd-client \
+  dovecot dovecot-lmtpd dovecot-pop3d dovecot-submissiond dovecot-pigeonhole-plugin rspamd-client xapian-core \
   && mkdir /var/lib/dovecot
 
+COPY --from=0 /usr/lib/dovecot/lib21_fts_xapian_plugin.* /usr/lib/dovecot/
+
 COPY conf /conf
 COPY start.py /start.py
 

core/dovecot/conf/dovecot.conf

@@ -21,7 +21,8 @@ mail_access_groups = mail
 maildir_stat_dirs = yes
 mailbox_list_index = yes
 mail_vsize_bg_after_count = 100
-mail_plugins = $mail_plugins quota quota_clone zlib
+mail_plugins = $mail_plugins quota quota_clone zlib fts fts_xapian
+default_vsz_limit = 2GB
 
 namespace inbox {
   inbox = yes
@@ -38,6 +39,12 @@ plugin {
   quota_vsizes = yes
   quota_clone_dict = proxy:/tmp/podop.socket:quota
 
+  fts = xapian
+  fts_xapian = partial=2 full=30
+  fts_autoindex = yes
+  fts_enforced = yes
+  fts_autoindex_exclude = \Trash
+
   {% if COMPRESSION in [ 'gz', 'bz2' ] %}
   zlib_save = {{ COMPRESSION }}
   {% endif %}