ownCloud server itself is running on port 8080 inside the docker container.
I would recommend to only expose the port and put a SSL terminating reverse proxy in front of it. That reverse proxy would have port 80 and 443 published, while ownCloud has no ports published.
I would spin up an ownCloud container in the same oC version as your RPi setup and look at the configuration and data directories.
Go to the RPi move the data directory to the same as inside the docker container: https://doc.owncloud.org/server/10.2/admin_manual/maintenance/manually-moving-data-folders.html
Then test-migrate the installation into the docker container. You could easily spin up a test container and try this process several times, until you have figured out all configuration details.
Make sure to remove all details of the previous containers (volumes, …) , when starting over.
Once it is all running, you can do a final migration.