ci(docker): add hadolint + shellcheck for container build inputs

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

.github/workflows/docker-lint.yml

@@ -0,0 +1,68 @@
+name: Docker / shell lint
+
+# Lints the container build inputs: Dockerfile (via hadolint) and any shell
+# scripts under docker/ (via shellcheck). These catch the class of regression
+# the behavioral docker-publish smoke test can't — unquoted variable
+# expansions, silently-failing RUN commands, etc.
+#
+# Rules and ignores are documented in .hadolint.yaml at the repo root.
+# shellcheck severity is pinned to `error` so SC1091-style "can't follow
+# sourced script" info-level warnings don't fail the job — the .venv
+# activate script doesn't exist at lint time.
+
+on:
+  push:
+    branches: [main]
+    paths:
+      - Dockerfile
+      - docker/**
+      - .hadolint.yaml
+      - .github/workflows/docker-lint.yml
+  pull_request:
+    branches: [main]
+    paths:
+      - Dockerfile
+      - docker/**
+      - .hadolint.yaml
+      - .github/workflows/docker-lint.yml
+
+permissions:
+  contents: read
+
+concurrency:
+  group: docker-lint-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  hadolint:
+    name: Lint Dockerfile (hadolint)
+    runs-on: ubuntu-latest
+    timeout-minutes: 5
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+
+      - name: hadolint
+        uses: hadolint/hadolint-action@54c9adbab1582c2ef04b2016b760714a4bfde3cf # v3.1.0
+        with:
+          dockerfile: Dockerfile
+          config: .hadolint.yaml
+          failure-threshold: warning
+
+  shellcheck:
+    name: Lint docker/ shell scripts (shellcheck)
+    runs-on: ubuntu-latest
+    timeout-minutes: 5
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+
+      - name: shellcheck
+        uses: ludeeus/action-shellcheck@00cae500b08a931fb5698e11e79bfbd38e612a38 # v2.0.0
+        env:
+          # Severity = error: SC1091 (can't follow sourced script) is info-
+          # level and would otherwise fail when the venv activate script
+          # doesn't exist at lint time.
+          SHELLCHECK_OPTS: --severity=error
+        with:
+          scandir: ./docker