Phase 0.5 of the s6-overlay supervision plan. Catches Dockerfile and
shell-script regressions that the behavioral docker-publish smoke test
can't surface — unquoted variable expansions, silently-failing RUN
commands, missing apt-get clean, etc.
Both lint clean against the current (tini) Dockerfile + entrypoint.sh
at the configured thresholds (hadolint: warning, shellcheck: error).
Each ignore in .hadolint.yaml carries a one-line justification; the
shellcheck severity floor is documented in the workflow file.
Refs: docs/plans/2026-05-07-s6-overlay-dynamic-subagent-gateways.md
