apk-cache(5)
/etc/apk/cache - local cache of package files
If /etc/apk/cache is a symlink to a local directory, apk(8) will use it as a local cache for downloaded package files and repository indicies. The cache must not reside on a tmpfs.
To enable the apk cache, run the following commands as root:
mkdir -p /var/cache/apk++ ln -s /var/cache/apk /etc/apk/cache
For information on cache maintenance, see apk-cache(8).