CI is not triggered for pull requests

Hi @AkihiroSuda !

I would love to take a shot at this but would like to know more about the desired behaviors.

To narrow it down:

Are you wanting all PRs against just the default shared branch (master, in this case)? Or every PR against every branch in the repository?
- I'm not sure what the project expects for maintaining multiple branches on this repo, so just want to check. Currently there is only one other branch existing.
Which workflows exactly do you have in mind to run on PRs? Just the make-all workflow? Or should make-image also be included?
Presumably the code that is built should be the code on the head branch?
- I'm sure this sounds like a silly question but I don't like to assume anything if I can help it!

genuinetools / img