Container user linuxgsm crontab can be saved optionally

[WildPenquin]

This should be transparent to users and requires no intervention if they do not wish to use a custom crontab. Current behavior is retained.

To actually use the custom crontab, the user needs to set LGSM_SAVE_CRONTAB to true. If set to false, the container will function exactly as it functions now. This variable needs to be documented in the downstream (docker image) repository.

[WildPenquin]

I've noticed there used to be similar pull requests previously. However this one is more simple and sometimes setting a crontab within the container is a requested feature. I don't personally need it and I understand if this is not desired for some reason (keeping the Dockerfile simple?).

This will save the crontab in /data/saved-crontab. It is not the cleanest way, and can be seen as a bit hacky, however as the crontabs are in /var/spool, I'm not sure how to make it in a cleaner way, save making a volume under /var/spool/ just for the sake of saving the crontab. And, some users might still be accustomed to changing UPDATE_CHECK.

[WildPenquin]

This would fix GameServerManagers/LinuxGSM#4584 .

[snowyyd]

It would also be a good idea to use something like supercronic, since that would allow us to run the container with stricter security options (e.g., no-new-privileges). Currently, crontab(1) relies on the SGID bit to edit crontabs(4) files.

[WildPenquin]

I've never heard of supercronic before and I'm not familiar with it, but after a quick glance it might be a good idea indeed.

[snowyyd]

Yes, it would be ideal to rely as little as possible on root (which currently runs cron in entrypoint.sh).