chore(docs): add additional information for using RES_OPTIONS and LOCALDOMAIN

[chobits]

Summary

By default, when Nginx master starts worker processes, it removes all environment variables. This prevents the use of $RES_OPTIONS and $LOCALDOMAIN. Therefore, this PR adds additional information to kong.conf.default to guide users on utilizing these variables effectively.

Checklist

• N/A The Pull Request has tests
• N/A A changelog file has been created under changelog/unreleased/kong or skip-changelog label added on PR if changelog is unnecessary. README.md
• N/A There is a user-facing docs PR against https://github.com/Kong/docs.konghq.com - PUT DOCS PR HERE

Issue reference

Fix #13301 and KAG-4845

[chobits]

A potential thorough fix would be to set these env variables in Kong's nginx configuration by default. However, for now, this documentation fix is simpler.

[hanshuebner]

Suggested change

	# they have been set. To use these environment variables, set the option
	# `nginx_main_env = RES_OPTIONS; env LOCALDOMAIN`. Otherwise, Nginx will
	# removes these environment variables when starting worker processes.
	# they have been set. To use these environment variables, set the options
	# `nginx_main_env = RES_OPTIONS;` and `env LOCALDOMAIN` in your nginx
	# configuration template to prevent nginx from removing them during worker
	# process start.

It seems to me that this suggestion, while workable, is pretty difficult. Is there a good reason not to put these directives into the default nginx template?

[chobits]

I originally thought this fix would be easier and more stable for 3.8 release, considering we have another DNS cname regression that needs fixing in next release (#13294). Therefore, I prefer not to involve additional code modification in the DNS client at this time.

[chobits]

OKay I'll research how to introduce environment variables into the default template of Kong later.

[Tieske]

isn't all there is to do, adding:

env RES_OPTIONS;
env LOCALDOMAIN;

to the template? It doesn't even have to be configurable, right?

[chobits]

That's it. Just like in your example, we just need to hard code them.