The current behavior of `docker swarm init` is to set up a swarm that
has no secret for joining, and does not require manual acceptance for
workers. Since workers may sometimes receive sensitive data such as pull
credentials, it makes sense to harden the defaults.
This change makes `docker swarm init` generate a random secret if none
is provided, and print it to the terminal. This secret will be needed to
join workers or managers to the swarm. In addition to improving access
control to the cluster, this setup removes an avenue for
denial-of-service attacks, since the secret is necessary to even create
an entry in the node list.
`docker swarm init --secret ""` will set up a swarm without a secret,
matching the old behavior. `docker swarm update --secret ""` removes the
automatically generated secret after `docker swarm init`.
Closes#23785
Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>
