Check for docker locally before registry login

We were checking before `kamal build push`, but not `kamal registry login`.
Since `kamal registry login` is called first by a deploy we don't
get the nice error message.

lib/kamal/cli.rb

@@ -2,6 +2,7 @@ module Kamal::Cli
   class BootError < StandardError; end
   class HookError < StandardError; end
   class LockError < StandardError; end
+  class DependencyError < StandardError; end
 end
 
 # SSHKit uses instance eval, so we need a global const for ergonomics