[chore](trx-rs): add Podman act_runner CI deployment

Add container/: a rootless Podman + systemd (Quadlet) setup to run
per-project Gitea Actions runners on one host instead of VMs. Uses the
host executor with a purpose-built image that bakes in the Rust toolchain
and all build dependencies (opus, alsa, soapysdr, clang), so CI runs skip
the per-run install and cold soapysdr-sys build.

Includes the runner Containerfile, first-boot registration entrypoint,
act_runner config template, two Quadlet units (trx-rs + a second project),
and a README covering build, registration, the required host-executor
workflow tweak, and tuning.

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

container/entrypoint.sh

@@ -0,0 +1,38 @@
+#!/usr/bin/env bash
+# SPDX-FileCopyrightText: 2026 Stan Grams <sjg@haxx.space>
+#
+# SPDX-License-Identifier: GPL-2.0-or-later
+#
+# Registers the runner on first boot (if no .runner state exists in /data),
+# then runs the act_runner daemon. Idempotent: on subsequent boots it reuses
+# the stored registration and ignores the token.
+set -euo pipefail
+
+CONFIG_FILE="${CONFIG_FILE:-/data/config.yaml}"
+
+cd /data
+
+# Seed the config from the image's template on first boot so it lives in the
+# persistent volume and can be edited there.
+if [ ! -f "$CONFIG_FILE" ]; then
+    cp /etc/act_runner/config.yaml "$CONFIG_FILE"
+fi
+
+# runner.file in config.yaml is ".runner" (relative to this CWD => /data/.runner).
+if [ ! -f /data/.runner ]; then
+    if [ -z "${GITEA_RUNNER_REGISTRATION_TOKEN:-}" ]; then
+        echo "ERROR: no /data/.runner registration and GITEA_RUNNER_REGISTRATION_TOKEN is empty." >&2
+        echo "       Grab a token from the repo's Settings -> Actions -> Runners and set it" >&2
+        echo "       in the Quadlet unit for the first boot only." >&2
+        exit 1
+    fi
+    echo "Registering runner '${GITEA_RUNNER_NAME:-podman}' with ${GITEA_INSTANCE_URL} ..."
+    act_runner register --no-interactive \
+        --config   "$CONFIG_FILE" \
+        --instance "${GITEA_INSTANCE_URL:?set GITEA_INSTANCE_URL}" \
+        --token    "$GITEA_RUNNER_REGISTRATION_TOKEN" \
+        --name     "${GITEA_RUNNER_NAME:-podman}" \
+        --labels   "${GITEA_RUNNER_LABELS:-ubuntu-latest:host}"
+fi
+
+exec act_runner daemon --config "$CONFIG_FILE"