Support or warn on .dockerignore

[schneems]

In #6 (reply in thread) it's mentioned that a customer was expecting their .dockerignore contents to be removed from the final image. Which makes sense to me. The experience can be improved.

It would be nice if either we determined this was a standard we want to support in CNBs where this is done automatically in lifecycle or if we want to support it somewhere else (such as a "heroku-warnings" buildpack we all depend on). I think it could either treat it as a file in the project.toml "exclude" list automatically or at a minimum warn "You have this file in your .dockerignore that is not in your project.toml 'exclude' list."

I was curious if there was prior discussion. I can't find it mentioned in

• The spec https://github.com/search?q=repo%3Abuildpacks%2Fspec%20.dockerignore&type=code

There is one reference in code in paketo-buildpacks https://github.com/paketo-buildpacks/jam/blob/14b6860a48a85e83d6b02f1c13f7508fb831f56f/internal/ihop/client_test.go#L132-L134 but github code search is also returning results in pull requests that don't actually include ".dockerignore" (via cmd+f) so maybe there are more https://github.com/search?q=org%3Apaketo-buildpacks+.dockerignore&type=pullrequests.

[schneems]

Also to add. Heroku currently has a .slugignore file that is applied after the build but before runtime. There is interest in giving CNB application owners a consistent API for removing artifacts that aren't needed at runtime.