Docker volumes are not initialized

According to https://docs.docker.com/engine/userguide/dockervolumes/:

Volumes are initialized when a container is created. If the container’s base image contains data at the specified mount point, that existing data is copied into the new volume upon volume initialization.

I just found that the volumes created by gitlab runner are actually empty, not initialized as above. Is this intended or is it a bug / missing feature? If intended, I'll be happy to submit a PR that updates docs/configuration/advanced-configuration.md.