As there is `apk upgrade` and `apk update`, the apk index will already
be existed. `--no-cache` is for `apk` when there is no `apk update`
behavior and it's expected to be no local cache left, not suitable for
the use case here, which wants to upgrade all the package to the latest
when packaging the image.
.. see also #1559.
It "mkdirs" the needed etc and bin directories first, then copies
stuff over. It also reduces a few layers.
Also it corrects a mistake in the Readme.md (docker exec --> run)
Thanks for the discussion @Alex131089
