Can't use with external HDD

I am trying to run a container and either have its entire container storage on an external hard drive or let it mount an external drive into the container, but I can’t tell if I am doing it right. I haven’t been able to find info on the former so I have been attempting the latter. My run command is

docker run -v /media/docs/mobile:/mobile nextcloud:17-apache

which I believe would allow my container at /mobile to mirror the storage on my linux filesystem at /media/docs/mobile. Is this correct?

It looks like it was a mounting error with the external drive.