radio: run liquidsoap as dedicated non-root user; quiet pulse warnings

- Liquidsoap refuses to start as root (init: security exit). Instead of
  the settings.init.allow_root override, the image now has a dedicated
  'radio' user (audio + pulse-access groups) and the entrypoint drops
  privileges via setpriv after doing its root-only work (volume seeding,
  icecast config render/start, pulse start, chown of the data volume).
  The icecast secret is made group-readable (640 root:radio) because the
  script parses it directly.
- The opam root moved from /root/.opam to /opt/opam so the liquidsoap
  binary AND its stdlib .liq files are readable by the radio user.
  NOTE: this invalidates the cached opam build layer - next build
  recompiles liquidsoap (~15-20 min).
- pulseaudio (PULSE_MODE=internal) now starts with
  --disallow-module-loading: system.pa startup modules still load, only
  later client-requested loads are blocked, and the system-mode warning
  goes away. The 'forcibly disabling SHM mode' notice is inherent to
  system mode and harmless (documented).

Keeps the user's libcurl4-gnutls-dev build-dep fix.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docker/Dockerfile.radio

@@ -47,10 +47,12 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
         icecast2 jq \
     && rm -rf /var/lib/apt/lists/*
 
-# Group membership for PulseAudio/ALSA access. On the Pi this was done by
-# hand for the system-wide pulse socket; in the image it is baked in. The
-# 'pulse-access' group comes with the pulseaudio package.
-RUN usermod -aG audio,pulse-access root
+# Liquidsoap refuses to run as root (security exit), so it gets a dedicated
+# user. audio/pulse-access mirror what was done by hand on the Pi for the
+# system-wide pulse socket ('pulse-access' comes with the pulseaudio pkg).
+RUN useradd --system --create-home --home-dir /var/lib/radio \
+        --shell /usr/sbin/nologin radio \
+    && usermod -aG audio,pulse-access radio
 
 # opam as a static binary so OPAM_VERSION is a real choice (apt would pin us
 # to whatever bookworm ships).
@@ -60,13 +62,16 @@ RUN ARCH=$(uname -m) \
     && chmod +x /usr/local/bin/opam
 
 # OCaml switch + liquidsoap and its optional feature libraries in one solve,
-# so liquidsoap is compiled WITH those features enabled.
+# so liquidsoap is compiled WITH those features enabled. OPAMROOT lives in
+# /opt/opam (not /root/.opam) so the unprivileged 'radio' user can read the
+# liquidsoap binary AND its stdlib .liq files at runtime.
+ENV OPAMROOT=/opt/opam
 RUN opam init -y --bare --disable-sandboxing \
     && opam switch create default "${OCAML_VERSION}" \
     && opam install -y "liquidsoap.${LIQUIDSOAP_VERSION}" ${LIQ_OPAM_PACKAGES} \
     && opam clean -a -c -s --logs
 
-ENV PATH="/root/.opam/default/bin:${PATH}"
+ENV PATH="/opt/opam/default/bin:${PATH}"
 
 # Build-time smoke test: the binary runs and reports the requested version.
 RUN liquidsoap --version

docker/entrypoint.radio.sh

@@ -73,7 +73,12 @@ fi
 #   none               - you edited the script to drop pulse in/out.
 case "${PULSE_MODE:-internal}" in
     internal)
-        pulseaudio --system --daemonize=yes --disallow-exit --exit-idle-time=-1 \
+        # --disallow-module-loading: modules from system.pa still load at
+        # startup; this only blocks later client-requested loads (and
+        # silences the system-mode warning). The "forcibly disabling SHM"
+        # notice is inherent to system mode and harmless.
+        pulseaudio --system --daemonize=yes --disallow-exit \
+            --disallow-module-loading --exit-idle-time=-1 \
             || echo "WARNING: internal pulseaudio failed to start" >&2
         export PULSE_SERVER="${PULSE_SERVER:-unix:/var/run/pulse/native}"
         ;;
@@ -84,5 +89,15 @@ case "${PULSE_MODE:-internal}" in
         ;;
 esac
 
+# Liquidsoap refuses to run as root (init: security exit), so hand the data
+# volume to the dedicated 'radio' user and drop privileges for the main
+# process. The script parses the icecast secret directly, so make that one
+# file group-readable for 'radio' (kept 640, root-owned).
+chown -R radio:radio "$DATA"
+chgrp radio "$CREDS" 2>/dev/null && chmod 640 "$CREDS" || true
+if ! setpriv --reuid radio --regid radio --init-groups -- test -r "$MUSIC"; then
+    echo "WARNING: music dir $MUSIC is not readable by the 'radio' user" >&2
+fi
+
 cd "$DATA"
-exec "$@"
+exec setpriv --reuid radio --regid radio --init-groups -- "$@"