I am doing more testing of my River4 Docker container on Linux and I think I have figured out the mapping of the /river4data volume in the container to a local directory on my Linux host server.
The change I had to make is to use the full directory path to a folder in my home directory. Here is what I have done:
Create a directory in my home directory called localstorage. (It could be called river4data)
Note the full path: /home/frank/localstorage
Executed the following command: docker run --name river4 -v /home/frank/localstorage:/river4data -d -p 1337:1337 fmcpherson/river4 to run the container.
The data, lists, and rivers directories were created by river4.js, and the lists directory is empty. I confirmed by entering docker logs river4 that river4.js was running with zero feeds.
I then copied the readinglist.opml file to the lists directory, but got a permission error, so I entered sudo cp readinglist.opml /home/frank/localstorage/lists/readinglist.opml.
I ran docker logs river4 a few times again and eventually river4.js picked up the readinglist.opml file and started processing the feeds. I then checked the /data/feeds directory and noted that all of the feed subdirectories were created.