tests/secrets-not-embedded.test.mjs: static assertions that the Dockerfile
embeds no secrets (no secret-bearing ARG/ENV, no secret-path or blanket COPY)
and .dockerignore excludes env/credential files; non-vacuous mutation probes
for every assertion; Docker-gated probe that builds the image with a marker
env file in the context and scans every layer + image config for secret
values.
