feat(docker): remove gosu from bundled image; s6-setuidgid handles privilege drop

The s6-overlay migration replaced every runtime use of gosu with
s6-setuidgid (in stage2-hook.sh, main-wrapper.sh, per-service run
scripts, and cont-init.d hooks), but the gosu binary itself was still
being copied into the image from tianon/gosu, and several comments
across the repo still pointed to it.

Image changes:
- Drop the FROM tianon/gosu:1.19-trixie AS gosu_source stage
- Drop the COPY --from=gosu_source /gosu /usr/local/bin/ layer
- Net: one fewer base-image pull, ~12-15 MB layer eliminated

Documentation/comment refresh (no behavior change):
- Dockerfile: update root-user rationale comment + cont-init.d comment
- docker/main-wrapper.sh: drop "pre-s6 contract (gosu drop)" reference
- docker-compose.yml: update UID/GID remap comment
- .hadolint.yaml: update DL3002 ignore rationale
- website/docs/user-guide/docker.md: privilege-drop helper is s6-setuidgid now
- hermes_cli/config.py: docker_run_as_host_user docstring

tools/environments/docker.py runs *arbitrary user images* via the
terminal backend, not the bundled Hermes image. It still needs SETUID/
SETGID caps so user images that use gosu/su/s6-setuidgid all work.
Renamed the cap-list constant _GOSU_CAP_ARGS → _PRIVDROP_CAP_ARGS and
updated comments to list s6-setuidgid alongside the others as examples.
The matching test (test_security_args_include_setuid_setgid_for_gosu_drop
→ test_security_args_include_setuid_setgid_for_privdrop) was renamed
and its docstring updated; behavior is unchanged.

Verification:
- hadolint clean against .hadolint.yaml
- shellcheck clean against all docker/ shell scripts
- Image rebuilt successfully (sha 1a090924ccea)
- Docker harness: 19 passed in 41.87s (every Phase 0 test + Phase 4
  per-profile-gateway lifecycle + container-restart reconciliation)
- tests/tools/test_docker_environment.py: 23 passed (rename did not
  break test discovery; pre-existing unrelated mock warning)

The plan document (docs/plans/2026-05-07-s6-overlay-dynamic-subagent-gateways.md)
intentionally retains its historical references to gosu — it describes
the pre-s6 entrypoint as background for understanding the migration.

Dockerfile

@@ -1,5 +1,4 @@
 FROM ghcr.io/astral-sh/uv:0.11.6-python3.13-trixie@sha256:b3c543b6c4f23a5f2df22866bd7857e5d304b67a564f4feab6ac22044dde719b AS uv_source
-FROM tianon/gosu:1.19-trixie@sha256:3b176695959c71e123eb390d427efc665eeb561b1540e82679c15e992006b8b9 AS gosu_source
 FROM debian:13.4
 
 # Disable Python stdout buffering to ensure logs are printed immediately
@@ -38,7 +37,6 @@ RUN tar -C / -Jxpf /tmp/s6-overlay-noarch.tar.xz && \
 # Non-root user for runtime; UID can be overridden via HERMES_UID at runtime
 RUN useradd -u 10000 -m -d /opt/data hermes
 
-COPY --chmod=0755 --from=gosu_source /gosu /usr/local/bin/
 COPY --chmod=0755 --from=uv_source /usr/local/bin/uv /usr/local/bin/uvx /usr/local/bin/
 
 WORKDIR /opt/hermes
@@ -121,8 +119,10 @@ RUN cd web && npm run build && \
 USER root
 RUN chmod -R a+rX /opt/hermes && \
     chown -R hermes:hermes /opt/hermes/.venv /opt/hermes/ui-tui /opt/hermes/node_modules
-# Start as root so the entrypoint can usermod/groupmod + gosu.
-# If HERMES_UID is unset, the entrypoint drops to the default hermes user (10000).
+# Start as root so the s6-overlay stage2 hook can usermod/groupmod and chown
+# the data volume. Each supervised service then drops to the hermes user via
+# `s6-setuidgid hermes` in its run script. If HERMES_UID is unset, services
+# run as the default hermes user (UID 10000).
 
 # ---------- Link hermes-agent itself (editable) ----------
 # Deps are already installed in the cached layer above; `--no-deps` makes
@@ -138,8 +138,8 @@ RUN uv pip install --no-cache-dir --no-deps -e "."
 COPY docker/s6-rc.d/ /etc/s6-overlay/s6-rc.d/
 
 # stage2-hook handles UID/GID remap, volume chown, config seeding,
-# skills sync — all the work the old entrypoint.sh did between
-# gosu-drop and `exec hermes`. Wired in as cont-init.d/01- so it
+# skills sync — all the work the old entrypoint.sh did before
+# `exec hermes`. Wired in as cont-init.d/01- so it
 # runs before user services start.
 #
 # 02-reconcile-profiles re-creates per-profile gateway s6 service