If /etc/apk/cache is a symlink to a local directory, apk(8) will 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).