Backup functionality for the Server manager

[NIREKI]

This PR adds a backup function to our existing server manager. #10

• added ark_backup volume in the base dir and var/backups/asa-server folder in the container for saving the backups
• using tar.gz for compressing and saving the /ShooterGame/Saved directory
• manager backup saves the gameworld and creates a backup
• manager restore: stops the server, lists all available backups& asks for which one to restore, starts the server again
• edited readme to display new functions
• slightly edited manager.sh for using -it option on docker exec

[azixus]

Fixes #10