[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/trx-rs-runner.container

@@ -0,0 +1,41 @@
+# SPDX-FileCopyrightText: 2026 Stan Grams <sjg@haxx.space>
+#
+# SPDX-License-Identifier: GPL-2.0-or-later
+#
+# Rootless Podman Quadlet for the trx-rs Gitea Actions runner.
+# Install to ~/.config/containers/systemd/trx-rs-runner.container then:
+#   systemctl --user daemon-reload
+#   systemctl --user start trx-rs-runner
+#
+# First boot only: paste a registration token (repo Settings -> Actions ->
+# Runners) into GITEA_RUNNER_REGISTRATION_TOKEN. After the runner appears
+# online you can blank it again — the registration is persisted in the volume.
+
+[Unit]
+Description=Gitea Actions runner — trx-rs
+After=network-online.target
+Wants=network-online.target
+
+[Container]
+Image=localhost/trx-rs-ci:latest
+ContainerName=gitea-runner-trx-rs
+# Persistent state: .runner registration, cache, workspaces.
+Volume=gitea-runner-trx-rs-data:/data:Z
+
+Environment=CONFIG_FILE=/data/config.yaml
+Environment=GITEA_INSTANCE_URL=https://git.haxx.space
+Environment=GITEA_RUNNER_NAME=trx-rs-podman
+Environment=GITEA_RUNNER_LABELS=ubuntu-latest:host
+Environment=GITEA_RUNNER_REGISTRATION_TOKEN=
+
+# Resource caps so a heavy Rust build here cannot starve the other project's
+# runner on the same host. Tune to your box.
+PodmanArgs=--cpus=4.0 --memory=6g
+
+[Service]
+Restart=always
+# A cold Rust build can be slow; don't let systemd consider startup failed.
+TimeoutStartSec=0
+
+[Install]
+WantedBy=default.target