The old version of "build-branches.sh" skipped the pushing of images when one of
them didn't change its sources. When looking at the Netbox branches I noticed
that the "master" branch only changes when there is a new release. Because we
build the new releases anyway in "build-latest.sh" that leaves "develop" and
"develop-*" which change regularly. The new script "build-next.sh" is responsible
for building "develop-*" as the next major release of Netbox. The build of
"develop" is moved to the Github Action build matrix. This has the additional
advantage of being faster because more builds are done in parallel.
