New enhancing on backups "START_WHEN_PLAYERS_JOIN"

I know that setting `PAUSE_IF_NO_PLAYERS=true` lets you pause backups if no players are online, and will start checking the server's player count every minute.

However, it's appeared that the back up process is starting immediately as soon as I started the server - which no player is entered yet - and cluttered the back up snapshot being made last time, plus for some modded server the waiting time before the server can start is very long, and I can't seem to config the 'right' "INITIAL_DELAY" very well.

So I'm suggest that maybe, if you includes another configurable env variable "START_WHEN_PLAYERS_JOIN=true" to let the backing up process start when first player join - so that I don't need to specify the INITIAL_DELAY and don't cluttered the back up instance, that'll be great!

Thank you very much.

Start by locating the existing handling for PAUSE_IF_NO_PLAYERS and INITIAL_DELAY in the backup container. Test behavior with no players online and then after the first player joins; done means START_WHEN_PLAYERS_JOIN can defer backup activity until that first join without requiring INITIAL_DELAY.

Written by the indexing model from the issue text.