Since Dovecot release 2.3.0 there is support for LUA-script-based passdbs and userdbs, however this support is currently not being build in alpine.

In order to enable LUA support just a few changes are necessary:

1. add the lua dev package to makedepends
2. add a lua subpackage
3. add —with-lua=plugin to the _configure call in build()
4. create the subpackage with the _lua

(from redmine: issue id 10215, created on 2019-04-08)