Are you using docker-machine? It is highly recommended that you do for local installations, and the answer to your question becomes very complicated if you are not.
If you are using docker-machine then you should have a virtualbox vm running in the background which is the docker-machine instance. The data is being copied within this virtualbox instance.
The first time the db container starts up all the data is copied to the /var/lib/docker/volumes/i2b2transmart-data/_data folder on this virtualbox vm before Oracle is started.
You can access this vm using 'docker-machine ssh ’ to work with the files themselves.
That being said, it is best to let the docker system handle all this copying of files. It will take you the same time to move it as it takes docker to unpack it. Additionally there is metadata that docker manages to know what state everything is in and manually creating the volume in the right state is more than just copying the files.
If you get the timeout error we have found that the process of copying is still happening in the background even after the error. If you come back later you should be able to ‘docker-compose up db’ and have the db container started without issue.
As a side note if you happen to have your power settings set too aggressively on the conservation side the machine may go to sleep during the copy which causes the timeout to have been exceeded but no work to actually have gotten done.
Hope this helps!