fix: install lua-unbound to silence DNS resolver fallback warning

Prosody 13 prefers lua-unbound for DNS but the image never installed it,
causing a startup warning and a fallback to the old resolver.

Part-of: <http://gitlab.vnc.biz/uxf/vnctalk-prosody/-/merge_requests/4>

Dockerfile

@@ -4,6 +4,7 @@ RUN apk update && apk upgrade && \
   apk add gettext lua5.4 lua5.4-socket lua5.4-dbi-postgresql lua5.4-expat lua5.4-sql-postgres \
   lua5.4-filesize lua5.4-lpeg lua5.4-hiredis lua5.4-filesystem lua5.4-ldap \
   lua5.4-sec lua5.4-lzlib lua5.4-cjson lua5.4-dev \
+  lua-unbound \
   libidn-dev libidn icu icu-dev libpq postgresql-dev make gcc expat expat-dev \
   libxmlb-dev libc-dev openssl openssl-dev htop nodejs nodejs-dev \
   libc-dev g++ yarn nagios-plugins-tcp patch && \
@@ -40,6 +41,7 @@ RUN apk update && apk upgrade && \
   apk add gettext lua5.4 lua5.4-socket lua5.4-dbi-postgresql lua5.4-expat lua5.4-sql-postgres \
   lua5.4-filesize lua5.4-lpeg lua5.4-hiredis lua5.4-filesystem lua5.4-ldap \
   lua5.4-sec lua5.4-lzlib lua5.4-cjson \
+  lua-unbound \
   libidn-dev libidn icu libpq postgresql-client expat \
   openssl nodejs nagios-plugins-tcp && \
   rm -rf /var/cache/apk/*