Docker.github.io: Add note to use stack deploy option --with-registry-auth

[myregistration]

File: get-started/part4.md, CC @johndmulhausen

Add a note in documentation for the env stack deploy command to use option --with-registry-auth when using images in a private repository or else the containers won't be created. Perhaps it's rare that someone makes their repository private, but it is possible.

docker stack deploy --with-registry-auth -c docker-compose.yml getstartedlab

[joaofnfernandes]

Thanks for the suggestion!

We strive to maintain the getting started simple and accessible. In this case it tells you to do a docker push <user>/<repo>:<tag>, and assumes that 99% of the users going through this are new to Docker and have never created a repository. So when users push, a new repository will be created with the default visibility setting - public.

As users explore more about Docker and create private repositories, they can use the docker stack deploy to learn how to authenticate against the private registry.

For that reason I'm closing this issue, but feel free to comment.