dev-mirror docker: create /etc/docker before writing daemon.json

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QKxGW1raAWaqeU8WdHsMsp

cn/lib/dev-mirror.sh

@@ -104,6 +104,7 @@ cn_dm_get_docker() {
 cn_dm_set_docker() {
   local url="$1" current="{}"
 
+  sudo mkdir -p /etc/docker
   if sudo test -f /etc/docker/daemon.json; then
     current=$(sudo cat /etc/docker/daemon.json)
   fi