ci: Reuse pre-built docker builder image for rapid local CI iteration testing #23344

Reuse the pre-built image, if available, to bypass most of ./ct/test/04_install.sh

ct/test/04_install.sh can run once without impacting the rest of the CI flow. This change reuses the docker image that was already built with installed external packages/software. Useful for changing CI flow or verify their current changes don't break CI build without the need to push to remote github.

This is a marginal improvement, it decreased my total time by about ~60 seconds for a local re-run. If you are iterating multiple times this can add to a decent amount of saved time.

Do I understand correctly that this change is actually useful only when running CI tasks locally?

Do I understand correctly that this change is actually useful only when running CI tasks locally?

Correct. It is a convenience for those who change CI flow and want to verify locally.

Looks fragile, as this will fail to update /ro_base and probably break other stuff?

Looks fragile, as this will fail to update /ro_base and probably break other stuff?

Yup, it would break ro_base and other more subtle stuff. I'll close this and re-think this concept. I do like re-using the container, it helped me speed up local verification/testing.