setup-apkrepos: version detection for edge

setup-apkrepos currently looks at /etc/alpine-release to determine the version to use when adding repositories. However the edge package of alpine-base will contain a "release" version sometimes, currently it is 3.17.0. This causes setup-apkrepos to add v3.17.0 repositories, even if the system is on edge. How can we improve the detection? Would checking the existing /etc/apk/repositories file for the main version be better?