[chore](trx-rs): add OpenRC service for act_runner on Alpine

The runner host is Alpine (OpenRC, no systemd). Add an OpenRC init script
for act_runner (supervise-daemon, depends on docker) plus a conf.d
example for running one instance per project, and rewrite the runner
section of the README with Alpine setup steps (apk docker, dedicated user
in the docker group, register, service install).

Assisted-By: Claude Code (claude-opus-4)
Claude-Session: https://claude.ai/code/session_01NFpGtGTWUEYXLwZeZs2RAV
Signed-off-by: Stan Grams <sjg@haxx.space>

container/act_runner.confd.example

@@ -0,0 +1,14 @@
+# SPDX-FileCopyrightText: 2026 Stan Grams <sjg@haxx.space>
+# SPDX-License-Identifier: GPL-2.0-or-later
+#
+# Per-instance settings for an act_runner OpenRC service.
+# Copy to /etc/conf.d/<service-name>, e.g. /etc/conf.d/act_runner.trx-rs
+# (the name must match the /etc/init.d/ symlink).
+
+# User that runs the daemon. Must be a member of the `docker` group.
+runner_user="act"
+
+# Per-instance state dir (holds the .runner registration) and config file,
+# so two runners on one host stay independent.
+runner_dir="/var/lib/act_runner/trx-rs"
+runner_config="/etc/act_runner/trx-rs.yaml"