Only rebuild images that could have changed

[scottwittenburg]

The goal here is to only rebuild images that might actually need to be rebuilt, based on what files changed. I suggested I would do this here, see that discussion for more background.

[scottwittenburg]

I tested this stuff with a different workflow and rules since the way we have this workflow now, it only runs when pushing to main. Here's an example workflow: https://github.com/spack/spack-infrastructure/actions/runs/6179687042/job/16775011917

In there if you look through the jobs, you can see that the gh-gl-sync build ran the conditional step, while others, like the gitlab-api-scrape one skipped that step.

I wish there were an early exit option for jobs, instead of having to put an if on every step after the check, but it seems not to exist. A lot of people want it though, based on the popularity of this issue.

[scottwittenburg]

It might be worth looking into whether we can use https://github.com/dorny/paths-filter instead (maybe that was the action you were referring to earlier).

It was actually this one I meant, which looks really active, is used by a lot of people, and has more features.

And then use the output from that action to build the job matrix dynamically

Yeah, I wanted to do that too, once I saw this workflow in change-actions repo, I wondered how we could do that in our case. Just need to figure out how to continue using our manually incremented tags in that scenario, but I didn't really try yet. I'll give it a go, as it would be nice to skip altogether the jobs we don't need.