Add configure-docker-builder.sh for Alpine vfs + bridge:none lab nodes.

Co-authored-by: Cursor <cursoragent@cursor.com>

sim/cluster0/scripts/configure-docker-builder.sh

@@ -0,0 +1,31 @@
+#!/bin/sh
+# Docker daemon settings for Alpine builder nodes (cast01+).
+# Alpine LTS kernel may lack overlay module; bridge creation can fail on lab VMs.
+# Use vfs storage + bridge:none + host networking in docker-build-runner.sh.
+set -eu
+ROOT="$(cd "$(dirname "$0")/.." && pwd)"
+# shellcheck source=/dev/null
+. "$ROOT/scripts/lib/common.sh"
+load_cluster_env
+
+DAEMON="/etc/docker/daemon.json"
+mkdir -p /etc/docker
+
+log "configure docker builder daemon → ${DAEMON}"
+cat > "${DAEMON}" <<'JSON'
+{
+  "iptables": false,
+  "ip6tables": false,
+  "bridge": "none",
+  "storage-driver": "vfs"
+}
+JSON
+
+rc-service docker restart 2>/dev/null || rc-service docker start 2>/dev/null || true
+sleep 2
+if docker info >/dev/null 2>&1; then
+  log "docker OK storage=$(docker info 2>/dev/null | awk -F': ' '/Storage Driver/{print $2}')"
+else
+  log "WARN docker info failed after restart"
+fi
+log "configure-docker-builder_ok $(host_short)"