Recommended way to create a shared volume within a docker managed volume?

If there is a directory such as /var/app/data with stateful information about a container and in that same directory there is a /var/app/data/config sub-directory that needs to be shared between a container cluster, how would you create the managed volumes for data and config?

If you absolutely cannot change directory structure then try using --mount

data volume /var/app/data
config volume /var/app/data/config

The readonly option, if present, causes the bind mount to be mounted into the container as read-only.