Dump Stack when docker fails on healthcheck

Send SIGUSR1 to dockerd to save stack dump of docker daemon
in order to be able to investigate why docker daemon was
unresposive to health check done by `docker ps`.

See https://github.com/moby/moby/blob/master/daemon/daemon.go on
how docker sets up a trap for SIGUSR1 with `setupDumpStackTrap()`

cluster/gce/gci/health-monitor.sh

@@ -50,6 +50,12 @@ function container_runtime_monitoring {
   while true; do
     if ! timeout 60 ${healthcheck_command} > /dev/null; then
       echo "Container runtime ${container_runtime_name} failed!"
+      if [[ "$container_runtime_name" == "docker" ]]; then
+          # Dump stack of docker daemon for investigation.
+          # Log fle name looks like goroutine-stacks-TIMESTAMP and will be saved to
+          # the exec root directory, which is /var/run/docker/ on Ubuntu and COS.
+          pkill -SIGUSR1 dockerd
+      fi
       systemctl kill --kill-who=main "${container_runtime_name}"
       # Wait for a while, as we don't want to kill it again before it is really up.
       sleep 120