truenas-apps-migration/general/add folder mapping for media.md

Add folder mapping for media

Assuming that the folder where media is stored is /mnt/tank/data1/Media:

1. on Truenas shell:
   jlmkr edit docker
2. add the following args to "systemd_nspawn_user_args":
   --bind='/mnt/tank/data1/Media:/mnt/media'