feat: use the js_periodic directive to check/issue/renew certs

[zsteinkamp]

Uses the new directive js_periodic in njs >= 0.8.2 to invoke clientAutoMode(r) on regular intervals. This eliminates the need to set up a cron or use docker healthchecks.

Proposed changes

• Use js_periodic to invoke clientAutoMode() periodically.
• Clarify some instructions for variable use in the shared dict zone.

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING document
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• I have updated any relevant documentation (README.md and CHANGELOG.md)

[ivanitskiy]

Please ensure integration tests are updated and tests both triggers "periodic" and regular HTTP request. it would require updating nginx config in integration tests

[4141done]

Something I don't understand: Can we have the ability to kickstart via a request as well as having the periodic process run? It seems like perhaps the js_periodic invocation could be in a normal location block that kicks it off and we could have both? Very possible that there's a situation I'm missing here but this change seems to take away some flexibility.

Should we add documentation here to show how to do it the old way?

js_content acme.clientAutoModeHTTP;

It could be that some users could prefer to invoke the check using an outside source as should in the deleted documentation.

[zsteinkamp]

Oh that's an interesting idea!

So we could do:

location = /acme/auto {
  js_periodic acme.clientAutoMode 1m;
  js_content acme.clientAutoModeHTTP;
}

Then people who were impatient could just hit it with curl. Ya?

[4141done]

Oh interesting I had not considered combining the two. Kind of strange the js_periodic needs to be in a location block to work since it's not tied to a location necessarily but 🤷 .

Yes I think this accomplishes the goal I was thinking of. Gives the user's automation the power to kickstart things on their desired cadence while still maintaining the automatic operation.
If we get requests for entirely manual management we could introduce a config option but until then this seems like a reasonable compromise 👍

[zsteinkamp]

Sweet. I've documented it as an option in the README.md. LMK what you think...