imagebuilder: fix APK for packages dir and cache

imagebuilder: fix APK for packages dir and cache

This commit solves multiple issues. First of just install the three
special packages base-files, libc and kernel directly from the index. In
upstream indexes, those will never appear to prevent accidental upgrades
may breaking the system.

Next, enable caching for the ImageBuilder, which speeds up consecutive
builds from ~33 seconds to ~5 seconds. Using cache however makes APK
create the folder `/var/cache/apk/` which conflicts with the base-files
installation, which ships a symlink from `/var` to `/tmp`, so specify
`--no-cache` for the rootfs initialization.

Lastly, drop the use of `apk update` since APK automatically does that.

Signed-off-by: Paul Spooren <mail@aparcar.org>