We need someway to pass the APFS volume number in to be included in the mount command.
I thought my drive was not mounting correctly because I couldn't find any files, but it turns out it correctly mounted an empty volume.
Here is the command I used to access my files, since selecting the volume is not presently possible:
/usr/bin/apfs-fuse -o uid=99,gid=100,allow_other,vol=1 /dev/sda /tmp/apfs-mount-folder/
The vol=1 specifies volume #1 (the second volume in the container)