audio-bridge: switch /run/dbus + /run/user/0 from bind-mount to tmpfs

The host's /run/dbus/system_bus_socket is the standard dbus socket for
systemd. Bind-mounting the container's socket onto /run/dbus overwrites
the host's socket, breaking dockerd's ability to manage cgroups via
systemd (manifests as 'org.freedesktop.systemd1 was not provided by any
.service files' on subsequent docker run).

Use tmpfs instead — ephemeral container-local sockets that don't conflict
with host services.

infra/compose.yaml

@@ -82,6 +82,9 @@ services:
   # Routes audio from librespot / mopidy / phone-Bluetooth to USB DAC (Q5K)
   # via ALSA. Falls back to RPi 3.5mm / HDMI if no USB DAC.
   # PEQ is applied on the DAC side (Q5K's Qudelix app, 20 bands), not here.
+  # NOTE: /run/dbus + /run/user/0 use tmpfs (NOT host bind-mount) — the host's
+  # dbus socket is /run/dbus/system_bus_socket and bind-mounting our container's
+  # socket overwrites it, breaking dockerd's ability to talk to systemd.
   audio-bridge:
     build: ./audio-bridge
     container_name: audio-bridge
@@ -90,9 +93,9 @@ services:
     network_mode: host
     devices:
       - /dev/snd:/dev/snd
-    volumes:
-      - /run/dbus:/run/dbus                # system DBus (bluetoothd, wireplumber)
-      - /run/user/0:/run/user/0            # PipeWire runtime dir
+    tmpfs:
+      - /run/dbus:size=1M
+      - /run/user/0:size=1M
     cap_add:
       - SYS_ADMIN                          # bluetoothd needs CAP_SYS_ADMIN for HCI
       - NET_ADMIN