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/registry.rb

@@ -3,6 +3,8 @@ class Kamal::Cli::Registry < Kamal::Cli::Base
   option :skip_local, aliases: "-L", type: :boolean, default: false, desc: "Skip local login"
   option :skip_remote, aliases: "-R", type: :boolean, default: false, desc: "Skip remote login"
   def login
+    ensure_docker_installed
+
     run_locally    { execute *KAMAL.registry.login } unless options[:skip_local]
     on(KAMAL.hosts) { execute *KAMAL.registry.login } unless options[:skip_remote]
   end