[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/project2-runner.container

@@ -0,0 +1,40 @@
+# SPDX-FileCopyrightText: 2026 Stan Grams <sjg@haxx.space>
+#
+# SPDX-License-Identifier: GPL-2.0-or-later
+#
+# Rootless Podman Quadlet for the SECOND project's Gitea Actions runner,
+# co-located on the same host as the trx-rs runner.
+#
+# It has its own name, its own data volume and its own registration token, so
+# the two runners are fully independent. They share the `ubuntu-latest` label,
+# but registration SCOPE (which repo each token came from) keeps their jobs
+# separate — neither will pick up the other's work.
+#
+# If project 2 needs different build dependencies, build it its own image from
+# an adjusted Containerfile and point Image= at that instead of reusing the
+# trx-rs image below.
+
+[Unit]
+Description=Gitea Actions runner — project 2
+After=network-online.target
+Wants=network-online.target
+
+[Container]
+Image=localhost/gitea-act-runner:latest
+ContainerName=gitea-runner-project2
+Volume=gitea-runner-project2-data:/data:Z
+
+Environment=CONFIG_FILE=/data/config.yaml
+Environment=GITEA_INSTANCE_URL=https://git.haxx.space
+Environment=GITEA_RUNNER_NAME=project2-podman
+Environment=GITEA_RUNNER_LABELS=ubuntu-latest:host
+Environment=GITEA_RUNNER_REGISTRATION_TOKEN=
+
+PodmanArgs=--cpus=4.0 --memory=6g
+
+[Service]
+Restart=always
+TimeoutStartSec=0
+
+[Install]
+WantedBy=default.target