Added waiting for backups to complete before stopping

[Dashboy1998]

Context

• Currently the container can stop while a backup is running

Choices

• Using pgrep instead of pidof as pidof only works for child processes
• Using a loop and array in the event multiple backups are running

Test instructions

1. Create a large file to force backup to take a while fallocate -l 10G palworld/Pal/Saved/deleteme
2. Start the container
3. Stop the server and while creating a backup docker exec -it palworld-server rcon-cli "shutdown 1" && docker exec -it palworld-server backup
4. Start the container
5. Run multiple backups
6. Stop the server/container
7. Start the container
8. Stop the container

Tip

If the container is being killed before backup is completed then try increasing stop_grace_period or decreasing the size of the large file.
If the backup is done too quickly to see that it's waiting then try increasing the size large file.

Checklist before requesting a review

• I have performed a self-review of my code
• I've added documentation about this change to the README.
• I've not introduced breaking changes.