AIAP - check if docker server is ready

* docker version returns both client and server version
  So even if server is not running, the client version will be returned
  and docker build will fail.
* So checking for containerd in docker version output. This will be true
  only if the server is also ready

Change-Id: Idd4355b4f8d3617a339df67ac9820e33311b7d79

tools/airship-in-a-pod/artifact-setup/assets/entrypoint.sh

@@ -47,7 +47,7 @@ function check_docker_readiness() {
   end=$(($(date +%s) + $timeout))
   echo "Waiting $timeout seconds for docker to be ready."
   while true; do
-    if ( docker version | grep 'Version' ); then
+    if ( docker version | grep 'containerd' ); then
       echo "docker is now ready"
       break
     else