build_local errored when the Procfile omitted the 'binary' field (on caution app build), though it is optional per the Procfile reference and the deploy path.
Mirror the "caution app deploy" path: branch on Option, falling back to build_enclave (full container filesystem) when absent
build_local errored when the Procfile omitted the 'binary' field (on `caution app build`), though it is optional per the Procfile reference and the deploy path.
Mirror the "caution app deploy" path: branch on Option<binary>, falling back to build_enclave (full container filesystem) when absent