[FEAT] customize: allow --server instead of --only-server

When creating a customized version of an image and including non-public plugins (as rocks), the implementation will use the `--only-server` flag to prevent it from reaching out to public LuaRocks servers.

Though this is a secure solution, it is also limiting. If the private plugin relies on external modules, it requires you to download all those rocks an store them locally to add to the local server.

By specifying `--server` instead of `--only-server` it would first try the local one, for the none public ones, and then if not found, try the public ones to fetch any dependencies.

This code: https://github.com/Kong/docker-kong/blob/b977b998f2a2b4fa0601a31b9d0ff21143769576/customize/packer.lua#L299