docker: opt in to dashboard --insecure via env var, never derive from bind host

The s6 dashboard run script flipped `--insecure` on whenever
`HERMES_DASHBOARD_HOST` was anything other than 127.0.0.1 / localhost.
That comment ("the dashboard refuses otherwise") predates the OAuth
auth gate: back when it was written, `start_server` would SystemExit
on any non-loopback bind, so the run script's `--insecure` was the
only way to make in-container deployments work at all.

The gate has since been replaced by `should_require_auth(host,
allow_public)`, which engages the OAuth flow when a
`DashboardAuthProvider` is registered (the bundled `dashboard_auth/nous`
provider auto-registers on `HERMES_DASHBOARD_OAUTH_CLIENT_ID`) and
fails closed with a specific operator-facing error when none is. The
host-derived `--insecure` ran upstream of all that and silently
disabled the gate on every container-deployed dashboard.

Most visible under the portal's wildcard-subdomain rollout: every Fly
machine binds 0.0.0.0 so the edge can reach Flycast, every machine
boots with the correct `HERMES_DASHBOARD_OAUTH_CLIENT_ID`, the nous
provider registers — and `/api/status` still returns
`{"auth_required": false, "auth_providers": ["nous"]}` because the
run script disabled the gate before `start_server` ever saw the
request. The dashboard SPA was served to anyone, no `/login` redirect,
no OAuth challenge.

Fix: derive `--insecure` from an explicit opt-in env var,
`HERMES_DASHBOARD_INSECURE` (truthy values matching the rest of the
s6 boolean envs: 1, true, TRUE, True, yes, YES, Yes). Operators on
trusted LANs behind a reverse proxy without the OAuth contract
(the existing `docker-compose.windows.yml` use case) opt in
explicitly; portal-managed agent deployments leave it unset and let
the gate engage.

`docker-compose.windows.yml` already passes `--insecure` on the
`command:` array directly (line 38), so it doesn't depend on the s6
auto-injection. No compose-file change required.

Tests:
* `tests/test_docker_home_override_scripts.py` — extends the existing
  static-text guard with a regression assertion that the legacy
  host-derived case-statement is gone and the new env-var opt-in is
  present (locks against accidental revert).
* `tests/docker/test_dashboard.py` — adds two Docker-in-Docker tests
  exercising the actual `/api/status` round-trip:
  - 0.0.0.0 bind + `HERMES_DASHBOARD_OAUTH_CLIENT_ID` → gate engaged
  - 0.0.0.0 bind + `HERMES_DASHBOARD_INSECURE=1` → gate disabled

Docs:
* `website/docs/user-guide/docker.md` + zh-Hans i18n — adds the new
  env var to the table, replaces the stale prose ("the entrypoint
  no longer auto-enables insecure mode" — which until this PR was
  flat-out wrong) with an accurate description of the gate's
  trigger conditions and the explicit opt-out.

shellcheck clean. Python static-text test passes locally. Behavioural
test will run against any future image build (CI's Docker harness).

docker/s6-rc.d/dashboard/run

@@ -30,13 +30,21 @@ cd /opt/data
 dash_host="${HERMES_DASHBOARD_HOST:-0.0.0.0}"
 dash_port="${HERMES_DASHBOARD_PORT:-9119}"
 
-# Binding to anything other than localhost requires --insecure — the
-# dashboard refuses otherwise because it exposes API keys. Inside a
-# container this is the expected deployment.
+# `--insecure` is opt-in via HERMES_DASHBOARD_INSECURE. The dashboard's
+# OAuth auth gate engages automatically on non-loopback binds when a
+# DashboardAuthProvider is registered (e.g. the bundled dashboard_auth/nous
+# provider, which auto-registers when HERMES_DASHBOARD_OAUTH_CLIENT_ID is
+# set). If no provider is registered, start_server fails closed with a
+# specific operator-facing error.
+#
+# This used to derive --insecure from the bind host ("anything non-loopback
+# implies insecure"), but that predates the OAuth gate and silently
+# disabled it on every container-deployed dashboard. The gate is now the
+# authority; operators on trusted LANs / behind a reverse proxy without
+# the OAuth contract opt in explicitly.
 insecure=""
-case "$dash_host" in
-    127.0.0.1|localhost) ;;
-    *) insecure="--insecure" ;;
+case "${HERMES_DASHBOARD_INSECURE:-}" in
+    1|true|TRUE|True|yes|YES|Yes) insecure="--insecure" ;;
 esac
 
 # shellcheck disable=SC2086  # word-splitting of $insecure is intentional