Allow `/etc/apk/cache` to be a symlink

[AkihiroSuda]

xx/src/xx-apk

Line 59 in 97118f5

if [ -d "/etc/apk/cache" ]; then

Any reason to disallow symlink here?

[crazy-max]

I think that's fine to check for symlink like: https://github.com/alpinelinux/alpine-make-vm-image/blob/247806967af479d39dcddd16eeda10556af5ee50/alpine-make-vm-image#L584-L587

[tonistiigi]

PR welcome