Even if your builders are downloading dependencies on the fly, you can and should force that through an artifact repository (e.g. artifactory) you control. They

Even if your builders are downloading dependencies on the fly, you can and should force that through an artifact repository (e.g. artifactory) you control. They shouldn't need arbitrary outbound Internet access. The builder needs a token injected with read-only pull permissions for a write-through cache and push permissions to the path it is currently building for. The only thing it needs to talk to is the artifactory instance.