Kill all cron

[priegger]

Is your feature request related to a problem? Please describe.
A lot is done using crontabs right now: polling docker for new images, time-based deployments, ...

Spacegun could be a lot simpler and easier to test if it did not have cron support but used hooks (webhooks) as triggers. That way git commits or docker pushes could directly trigger actions and no polling would be necessary. In case of time-based deployments, web cron or the system crontab and the cli can be used.

Describe the solution you'd like

• Get rid of cron, replace it with web hooks for all necessary triggers.
• Provide documentation for the hooks and examples, how to use web cron or maybe Docker containers with cron to trigger actions.

Describe alternatives you've considered
Keep the complexity:

• Keep cron and fix the flaky tests.
• Provide the hooks as an alternative to the cron feature.

Additional context

• It might be nice to have a well documented external cron tool. Maybe a crontab (file) with a default configuration or a docker container which triggers the hooks using the cli or a web endpoint.
• Authentication for some of the hooks will be necessary (trigger, which image should be deployed).