Add Dockerfile.opam-test for testing opam repo install workflow
Tests installing all monorepo packages from the opam overlay repo
on an oxcaml 5.2.0+ox switch. Documents known oxcaml incompatibilities
(ocamlformat-lib, js_of_ocaml-lwt, day10 ocaml>=5.3).
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>