The zsh triggers run fine:

alpine64:~$ cat /etc/shells
# valid login shells
/bin/sh
/bin/ash
/bin/dash
#/bin/bash
#/bin/lash
#/bin/msh

/bin/zsh

However they don’t show up in apk info:

$ apk info -dt zsh
zsh-5.0.0-r0 description:
A very advanced and programmable command interpreter (shell) for UNIX

zsh-5.0.0-r0 triggers:

alpine64:~$ apk --version
apk-tools 2.3.4, compiled for x86_64.

And what’s more, apk seems to only show a few directories for a few packages, not sure where from:

$ for f in $(apk info); do apk info -t $f; done|grep -v triggers:|sed '/^$/d'
/bin
/usr/bin
/sbin
/usr/sbin
/lib/modules/*
/lib
/usr/lib
/usr/share/kernel/*
/boot

(from redmine: issue id 1790, created on 2013-04-18, closed on 2013-04-18)