Optionally Exclude Media Files from Backup

https://github.com/AzuraCast/AzuraCast/issues/1020

"When using the backup feature of the docker.sh to backup to network storages the backup takes a large amount of time when there are a lot of media files. I want to be able to use AzuraCasts backup feature to make fast backups in order to get the whole stack setup when needing to use a backup.

To be able to backup AzuraCast regularly to a network storage without having to transfer large amounts of data every time it would be helpful to be able to exclude the media files from the .tar.gz that the backup command generates. Preferably via an optione like --no-media or something like that. With this it would be possible to make fast backups of AzuraCast and manually backup the media files via an rsync cron that only writes deltas to previously backed up media files."