Option to run once without cron

[Akruidenberg]

I would like to intergrate this container with with existing prehooks. Therefore, it would be nice to run the container once without a cron job,

[RealOrangeOne]

This is possible, but apparently undocumented. If you set $SCHEDULE to an empty string, it'll run the backup just once and exit.

This probably wants adding to the README