This command makes sure that serverA will accept ssh connection from serverB.
This is useful when you want your code to be automatically deployed to dokku, once you commit it to git hosting. Once you configure you
post-receive git hook to push code to dokku, you need to make sure that dokku server will authenticate the git hosting server.
cat public_key | sudo sshcommand acl-add dokku [anytext]
This command is executed on dokku server.
public_key is taken from git hosting (Phabriactor in my case).
dokku is no-login username.