Adds the /cache volume and set it to writeable by all. This allows for
those using jellyfin to continue using it without modifying their
config. However, retaining cache will require one to mount the /cache
volume.
Also make the /config and /media dirs 777 by default. No permissions on
mounted volumes will be changed.
By moving the apt-get layer before the copies, the apt-get layer can be reused with each build, reducing upload/download needed during updates.
Just a small optimization.
