Add only a part of an Alpine package

A coworker recently shelled into an Alpine-based container and ran apk add with some special additional syntax such that only a part of a package was installed. More specifically, he wanted only the command-line client of the redis package (which includes both client and server). I've scoured the web, and read through all of the relevant [man pages](https://man.archlinux.org/listing/extra/apk-tools/) , but I can't seem to find the invocation that he used.