feat(mdns): optional Avahi .local responder (ENABLE_MDNS)

Self-contained host-network Avahi container publishes the service names over
mDNS so .local works zero-config for guests (macOS/Linux native, Windows needs
Bonjour). Toggle with ENABLE_MDNS=true + BASE_DOMAIN=*.local; build-stack layers
docker-compose.avahi.yml in and builds the image during --prep. mDNS can't serve
the air-gap upstream spoofs (non-.local) — those still need real DNS.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docker/avahi/Dockerfile

@@ -0,0 +1,8 @@
+# Self-contained mDNS responder: publishes the stack's service names on .local
+# so guests resolve them with zero DNS/client config (macOS/Linux native;
+# Windows needs Bonjour). Used only when ENABLE_MDNS=true and BASE_DOMAIN=*.local.
+FROM alpine:3.20
+RUN apk add --no-cache avahi dbus
+COPY entrypoint.sh /entrypoint.sh
+RUN chmod +x /entrypoint.sh
+ENTRYPOINT ["/entrypoint.sh"]

docker/avahi/entrypoint.sh

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+#!/bin/sh
+# Publish A records for the stack's service names over mDNS (.local).
+# Requires host networking (mDNS is link-local multicast). One avahi-publish
+# per name; if any dies the container exits so Docker restarts it.
+set -eu
+
+: "${BASE_DOMAIN:?BASE_DOMAIN unset}"
+: "${HOST_LAN_IP:?HOST_LAN_IP unset}"
+
+case "$BASE_DOMAIN" in
+  *.local) ;;
+  *) echo "WARN: mDNS resolves only .local — BASE_DOMAIN=$BASE_DOMAIN will NOT be discoverable via avahi. Set BASE_DOMAIN=*.local to use mDNS." >&2 ;;
+esac
+
+mkdir -p /run/dbus
+rm -f /run/dbus/pid /run/dbus/system_bus_socket 2>/dev/null || true
+dbus-daemon --system
+for _ in 1 2 3 4 5; do [ -S /run/dbus/system_bus_socket ] && break; sleep 1; done
+
+avahi-daemon --daemonize --no-chroot
+sleep 2
+
+pids=""
+for n in "$BASE_DOMAIN" "auth.$BASE_DOMAIN" "packwiz.$BASE_DOMAIN" "mc.$BASE_DOMAIN"; do
+  echo "mDNS publish: $n -> $HOST_LAN_IP"
+  avahi-publish -a "$n" "$HOST_LAN_IP" &
+  pids="$pids $!"
+done
+
+# Block on the publishers; exit (→ restart) if one drops.
+# shellcheck disable=SC2086
+wait -n $pids 2>/dev/null || wait