See https://wiki.alpinelinux.org/wiki/Creating_an_Alpine_package

apk add alpine-sdk
git clone https://gitlab.alpinelinux.org/alpine/aports
adduser build
addgroup build abuild
mkdir -p /var/cache/distfiles
chmod a+w /var/cache/distfiles
chgrp abuild /var/cache/distfiles
chmod g+w /var/cache/distfiles
su - build
abuild-keygen -a -i