Added support for cron expression

[zinen]

I wanted to define some on and off time for this and are now testing with this code.

Not fully tested yet. But are you interessed?

Added to docker-compose.yml with this PR:

      - SCHEDULE_CRON=                               ## (optional) Scheuler in cron schedule expression 
      ## Example of cron schedule expression:
      #¤ - SCHEDULE_CRON=*/1 12-14 * * 1-5     = allowed to run every 1 minute at hours 12-14 on monday to friday
      ## more help with formatting at https://crontab.guru/#*/1_12-14_*_*_1-5

[Der-Henning]

Hi @zinen.
Thank you for your effort. This seems to be a useful feature and I will be happy to include it.
Please make sure that it is purely optional and it is possible to update without changing an existing config.
I haven't tested it yet but I will do so in the next days. I will create annotations if I see any issues.

[zinen]

Please make sure that it is purely optional and it is possible to update without changing an existing config. I haven't tested it yet but I will do so in the next days. I will create annotations if I see any issues.

Agreed. So far i should have input default settings make this setting both optional and so also seemless upgradeble for exsisting users without need to change config.

[zinen]

A few changes due to [5cfeb2a] should trigger a new review from you.

[Der-Henning]

Please note these annotations.

[zinen]

Marked PR ready for merge