Yeah, I was sick of having to write build scripts in vendor specific formats and then not being able to run those builds locally without jumping through a lot of hoops.

If earthly didn't exist I think I would use a combination of make inside docker files to define my build and then use the bare minimum of CI specific configs to just call into those. That way when I need to switch from Travis to Github or Circle or whatever, I don't need to rewrite the steps.