Add option for a cron based schedule

[etzelc]

Run the service updates specified by a cron schedule.

An environment variable SLEEP_SCHEDULE is introduced, which has to be a valid unix cron schedule of five fields. Example 0 8,16 * * * --> every day at 08:00 and 16:00

If SLEEP_SCHEDULE is set, any value in SLEEP_TIME is ignored unless the calculated next schedule time is an implausible value. Then it defaults to SLEEP_TIME.

Because I have not found the Korn Shell ksh93 in an alpine repository it is compiled and copied to the final image during the docker multi-stage build.

[etzelc]

On my local machine the arm64 build is successful. Maybe an issue on the CI machine?

[etzelc]

• Made the script more robust against miscalculations of the next scheduled run.
• Use the currently latest version (v1.0.4) of ksh93 instead of the dev branch.
• Optimized git clone to only load used tag and no history
• Additionally I tried to use Bash to avoid the arm64 CI build fail, but as you can see not successful.