I don't know how easy the following is to do with Unraid, but here's an example of this working:
# mkdir -p /root/test/mnt
# docker run -it --privileged --mount type=bind,source=/root/test,target=/test,bind-propagation=rshared --device /dev/fuse -v /root/repo:/repo b3vis/borgmatic bash
f069c33ce2cc:/# borg mount /repo/repo.borg::test /test/mnt
After that, the mounted archive show up within /test/mnt on the host. The main downside is you need to the run container as privileged. There might be a way to make this work with --add-cap instead, but that's beyond my Docker skills.
More background here: https://stackoverflow.com/a/53632103