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I want to mount my usb drive into a running docker instance for manually backup of some files.

I know of the -v feature of docker run, but this creates a new container. Note: its a nextcloudpi container.

Answers

You can only change a very limited set of container options after a container starts up. Options like environment variables and container mounts can only be set during the initial docker run or docker create. If you want to change these, you need to stop and delete your existing container, and create a new one with the new mount option.

If there's data that you think you need to keep or back up, it should live in some sort of volume mount anyways. Delete and restart your container and use a -v option to mount a volume on where the data is kept. The Docker documentation has an example using named volumes with separate backup and restore containers; or you can directly use a host directory and your normal backup solution there. (Deleting and recreating a container as I suggested in the first paragraph is extremely routine, and this shouldn't involve explicit "backup" and "restore" steps.)

If you have data that's there right now that you can't afford to lose, you can docker cp it out of the container before setting up a more robust storage scheme.