Auto-generate list of modified dockerfiles in main runner

Currently we hard-code Ocean 5 dockerfiles, see 8f1e5dd48b0696a181d64485333b89a51228c807.

We'll need to emulate `path-filtering` orb behavior and generate the list of modified dockerfiles from `git diff --name-only <head> <base>` (base being PR/branch parent, or merge commit parent), in order to dynamically set build parameter matrix.