docker: add audio passthrough for containerized FM receiver

New files for running the FM receiver in Docker with audio output:

- Dockerfile.gnuradio-audio: GNU Radio image with ALSA→PulseAudio bridge
  - libasound2-plugins for ALSA PulseAudio plugin
  - /etc/asound.conf configures ALSA to route to PulseAudio

- docker-compose.fm-receiver.yml: Full FM receiver setup
  - PulseAudio socket mount for audio
  - USB passthrough for RTL-SDR (requires privileged mode)
  - XML-RPC port 8090 exposed for tuning control
  - Environment vars: FREQ_MHZ, GAIN

- entrypoint-fm.sh: Builds and runs flowgraph at specified frequency
- run-fm-receiver.sh: Helper script with usage instructions

Usage:
  HOST_UID=$(id -u) FREQ_MHZ=107.2 docker compose -f docker/docker-compose.fm-receiver.yml up

docker/Dockerfile.gnuradio-audio

@@ -0,0 +1,35 @@
+FROM librespace/gnuradio:latest
+
+# PulseAudio client + ALSA-PulseAudio plugin for audio output
+# GNU Radio uses ALSA backend, so we bridge ALSA → PulseAudio
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    pulseaudio-utils \
+    libpulse0 \
+    libasound2-plugins \
+    alsa-utils \
+    && rm -rf /var/lib/apt/lists/*
+
+# Configure ALSA to use PulseAudio as default output
+# This makes any ALSA app (including GNU Radio) output to PulseAudio
+RUN echo 'pcm.!default { type pulse }' > /etc/asound.conf && \
+    echo 'ctl.!default { type pulse }' >> /etc/asound.conf
+
+# Create non-root user for PulseAudio (matches typical host UID)
+ARG USER_ID=1000
+ARG GROUP_ID=1000
+RUN groupadd -g ${GROUP_ID} gnuradio || true \
+    && useradd -m -u ${USER_ID} -g ${GROUP_ID} gnuradio || true
+
+WORKDIR /flowgraphs
+
+# PulseAudio socket location
+ENV PULSE_SERVER=unix:/run/pulse/native
+
+# XML-RPC port for runtime control
+ENV XMLRPC_PORT=8090
+EXPOSE 8090
+
+USER gnuradio
+
+# Default: run a flowgraph passed as argument
+ENTRYPOINT ["python3"]

docker/docker-compose.fm-receiver.yml

@@ -0,0 +1,32 @@
+services:
+  fm-receiver:
+    build:
+      context: .
+      dockerfile: Dockerfile.gnuradio-audio
+
+    # Root required for USB device access (RTL-SDR)
+    user: root
+    privileged: true
+
+    volumes:
+      # PulseAudio socket for audio output (ALSA → PulseAudio bridge)
+      - /run/user/${HOST_UID:-1000}/pulse/native:/run/pulse/native
+      # Flowgraph source files
+      - ../examples:/flowgraphs:ro
+      - ../src:/src:ro
+      # Entrypoint script
+      - ./entrypoint-fm.sh:/entrypoint-fm.sh:ro
+
+    environment:
+      - PULSE_SERVER=unix:/run/pulse/native
+      - FREQ_MHZ=${FREQ_MHZ:-101.1}
+      - GAIN=${GAIN:-10}
+
+    # XML-RPC port for tuning control from host
+    ports:
+      - "8090:8090"
+
+    entrypoint: ["/bin/bash", "/entrypoint-fm.sh"]
+
+    stdin_open: true
+    tty: true

docker/entrypoint-fm.sh

@@ -0,0 +1,22 @@
+#!/bin/bash
+# Entrypoint for containerized FM receiver
+set -e
+
+FREQ_MHZ=${FREQ_MHZ:-101.1}
+GAIN=${GAIN:-10}
+
+python3 -c "
+import sys, subprocess, os
+sys.path.insert(0, '/flowgraphs')
+sys.path.insert(0, '/src')
+from fm_scanner import build_fm_receiver
+
+freq = float(os.environ.get('FREQ_MHZ', '101.1'))
+gain = int(os.environ.get('GAIN', '10'))
+
+print(f'Building FM receiver for {freq} MHz (gain {gain} dB)...')
+py_path = build_fm_receiver(freq, gain=gain)
+print(f'Launching {py_path.name} — Ctrl+C to stop')
+print(f'XML-RPC control at http://localhost:8090')
+subprocess.run([sys.executable, str(py_path)])
+"

docker/run-fm-receiver.sh

@@ -0,0 +1,24 @@
+#!/bin/bash
+# Run containerized FM receiver with audio output to host
+#
+# Usage: ./run-fm-receiver.sh [FREQ_MHZ] [GAIN]
+#   FREQ_MHZ: FM frequency (default: 101.1)
+#   GAIN: RF gain in dB (default: 10)
+#
+# Once running, use XML-RPC from host to retune:
+#   python -c "import xmlrpc.client; p=xmlrpc.client.ServerProxy('http://localhost:8090'); p.set_freq(107.2e6)"
+
+set -e
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+
+export HOST_UID=$(id -u)
+export FREQ_MHZ=${1:-101.1}
+export GAIN=${2:-10}
+
+echo "Starting FM receiver at $FREQ_MHZ MHz (gain: $GAIN dB)"
+echo "XML-RPC control available at http://localhost:8090"
+echo "Press Ctrl+C to stop"
+echo
+
+docker compose -f "$SCRIPT_DIR/docker-compose.fm-receiver.yml" up --build