[fix] correct how repositories urls are saved

[lyz-code]

When I tried to run ./mkimage-alpine.sh -r v3.7 the script saved the repo url in /etc/apk/repositories as:

-e http://nl.alpinelinux.org/alpine/v3.2/main

Therefore the fetch once you run a docker fails.

With my fix it works again :)

[firecyberice]

The official alpine image is now multiarch so this is not needed anymore.