fix: mount containerd socket into alloy for cadvisor per-container stats

Nodes using Docker's containerd snapshotter (storage driver 'overlayfs') make
the cadvisor exporter open a containerd client to read container info; without
/run/containerd/containerd.sock it fails to register existing containers, so no
per-container metrics appear (only the machine root). Mount the socket read-only
— harmless on overlay2 nodes, required on containerd-snapshotter ones.

compositions/apex/docker-compose.yml

@@ -199,6 +199,7 @@ services:
       - ${APEX_COMMONS}/assets/alloy/config.alloy:/etc/alloy/config.alloy:ro
       - ${APEX_TIER2}/alloy/data:/var/lib/alloy
       - /var/run/docker.sock:/var/run/docker.sock:ro
+      - /run/containerd/containerd.sock:/run/containerd/containerd.sock:ro   # cadvisor per-container stats on nodes using the containerd snapshotter (overlayfs)
       - /proc:/host/proc:ro
       - /sys:/host/sys:ro
       - /:/rootfs:ro