invalidate stage builds only by the files they use

[turadg]

It's convenient to have all the files for the proposal at its top level, but because the Docker build doesn't know which will be needed for any stage, it has to copy them all. That entails invalidating the non-test stages when tests update.

Design

Files used only in testing shouldn't be copied before the test stage.

Use the new COPY --exclude to exclude *test* files before the test stage.

Consider doing the same for "use" stage with, perhaps, a *use* pattern.

Acceptance criteria

Modifying "test" files only invalidates the test image, not the prepare, exec, use, etc

Bonus: modifying "use" files only invalidates the use stage of the use image, not the prepare, exec, etc