If you want to add an external library (like an smb share that already has your pictures) then you will need to edit line 23 under volumes (- /mnt/smb_share_pics:/mnt/smb_share_pics:ro)
You will want to first mount said folder on the server. See https://wiki.gabesville.com/books/linux/page/mount-smb-share for instructions
Custom upload location, will want to mount /etc/fstab with extra settings: \