need a repository URL with SSL certificate
the current repository URL http://dl-3.alpinelinux.org/alpine/v3.4/main
is not supported by nexus repository with a reverse proxy setting as it
only takes Https URL for proxy repo.
So if we have another repository url with SSL certification included we
can use nexus repository with no blockers
(from redmine: issue id 7698, created on 2017-08-16)