[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/config.yaml

@@ -0,0 +1,32 @@
+# SPDX-FileCopyrightText: 2026 Stan Grams <sjg@haxx.space>
+#
+# SPDX-License-Identifier: GPL-2.0-or-later
+#
+# act_runner configuration template. Seeded into /data/config.yaml on first
+# boot; edit the copy inside the volume to change settings per runner.
+
+log:
+  level: info
+
+runner:
+  # Registration state. Relative to the daemon's working directory (/data).
+  file: .runner
+  # Concurrent jobs this runner will pick up. Rust builds are heavy — keep this
+  # modest, especially if two runners share one host. The trx-rs workflow has
+  # three parallel jobs (lint, test, reuse); capacity 2 lets two overlap.
+  capacity: 2
+  timeout: 3h
+  # Map the workflow's `runs-on: ubuntu-latest` to the HOST executor, i.e. run
+  # steps directly inside THIS container (which already has all the toolchain).
+  # No Docker/Podman socket is required in this mode.
+  labels:
+    - "ubuntu-latest:host"
+
+cache:
+  # Built-in actions cache server (used by actions/cache). Stored in the volume.
+  enabled: true
+  dir: "/data/cache"
+
+host:
+  # Where per-job workspaces are created.
+  workdir_parent: /data/workflows