Document more default variables in Actions environment

[janbrasna]

Code of Conduct

• I have read and agree to the GitHub Docs project's Code of Conduct

What article on docs.github.com is affected?

https://docs.github.com/en/actions/reference/workflows-and-actions/variables#default-environment-variables

What part(s) of the article would you like to see updated?

According to actions/runner-images#7879 (comment) there are more default variables populated with values not available elsewhere.

I'd like to see some commitment to the env var stability of these by documenting them publicly, as a part of the feature.

Additional information

The GHA implicit "Set up job" step has a lot of valuable info in the log. A lot of these values would be helpful to access and print later on, use as caching keys, mention in user-facing error messages etc. — however are not available in any variable, context or expression:

• Suggested cache key for OS release version? actions/cache#1025 (comment)

Some previously undocumented values have been mentioned by GHA staff:

• Add details about Runner OS version in Actions environment actions/runner-images#7879

Can some SME chime in and consider documenting more similarly useful values? Thanks.

[Sharra-writes]

@janbrasna Thanks for opening an issue! Let me check in and see if there's a reason we haven't done this beyond, maybe, not being able to keep up with changes.

[janbrasna]

Thank you so much! In case I haven't made the description exactly actionable, I'm mostly after the super-interesting $ImageOS and $ImageVersion — that have been apparently working for years, but are not part of any documented feature or area.

Debug run +print (expand for details)