Auto-retrieve /etc/apk/keys and /etc/apk/packages

[johnp]

On a non-alpine machine these do not exist and the requirements as mentioned in the README and project docs are misleading in that they only do not mention this requirement.

Keys could be retrieved from https://alpinelinux.org/keys/, though not all of them are in /etc/apk/keys.

For repositories it would be good to have a fallback (http://dl-cdn.alpinelinux.org/alpine/latest-stable/main ?) or use the setup-apkrepos script somehow.