Move docker login into build command

We only need to run the docker login commands for pushing and pulling
images.

So let's move the logins into those commands. This ensures we are logged
in when calling `kamal build` commands directly.

Fixes: https://github.com/basecamp/kamal/issues/919

lib/kamal/cli/build.rb

@@ -15,6 +15,8 @@ class Kamal::Cli::Build < Kamal::Cli::Base
     cli = self
 
     ensure_docker_installed
+    login_to_registry_locally
+
     run_hook "pre-build"
 
     uncommitted_changes = Kamal::Git.uncommitted_changes
@@ -61,6 +63,8 @@ class Kamal::Cli::Build < Kamal::Cli::Base
 
   desc "pull", "Pull app image from registry onto servers"
   def pull
+    login_to_registry_remotely
+
     if (first_hosts = mirror_hosts).any?
       #  Pull on a single host per mirror first to seed them
       say "Pulling image on #{first_hosts.join(", ")} to seed the #{"mirror".pluralize(first_hosts.count)}...", :magenta
@@ -181,4 +185,16 @@ class Kamal::Cli::Build < Kamal::Cli::Base
         execute *KAMAL.builder.validate_image
       end
     end
+
+    def login_to_registry_locally
+      run_locally do
+        execute *KAMAL.registry.login
+      end
+    end
+
+    def login_to_registry_remotely
+      on(KAMAL.hosts) do
+        execute *KAMAL.registry.login
+      end
+    end
 end

lib/kamal/cli/main.rb

@@ -20,9 +20,6 @@ class Kamal::Cli::Main < Kamal::Cli::Base
     runtime = print_runtime do
       invoke_options = deploy_options
 
-      say "Log into image registry...", :magenta
-      invoke "kamal:cli:registry:login", [], invoke_options.merge(skip_local: options[:skip_push])
-
       if options[:skip_push]
         say "Pull app image...", :magenta
         invoke "kamal:cli:build:pull", [], invoke_options
@@ -52,7 +49,7 @@ class Kamal::Cli::Main < Kamal::Cli::Base
     run_hook "post-deploy", secrets: true, runtime: runtime.round.to_s
   end
 
-  desc "redeploy", "Deploy app to servers without bootstrapping servers, starting kamal-proxy, pruning, and registry login"
+  desc "redeploy", "Deploy app to servers without bootstrapping servers, starting kamal-proxy and pruning"
   option :skip_push, aliases: "-P", type: :boolean, default: false, desc: "Skip image build and push"
   def redeploy
     runtime = print_runtime do