Docker login if exec might pull image

The `app exec` and `accessory exec` commands will run `docker run` if
they are not set to reuse existing containers. This might need to pull
an image so let's make sure we are logged in before running the command.

Fixes: https://github.com/basecamp/kamal/issues/1163

lib/kamal/cli/accessory.rb

@@ -152,6 +152,7 @@ class Kamal::Cli::Accessory < Kamal::Cli::Base
 
       when options[:interactive]
         say "Launching interactive command via SSH from new container...", :magenta
+        on(accessory.hosts.first) { execute *KAMAL.registry.login }
         run_locally { exec accessory.execute_in_new_container_over_ssh(cmd) }
 
       when options[:reuse]
@@ -164,6 +165,7 @@ class Kamal::Cli::Accessory < Kamal::Cli::Base
       else
         say "Launching command from new container...", :magenta
         on(hosts) do |host|
+          execute *KAMAL.registry.login
           execute *KAMAL.auditor.record("Executed cmd '#{cmd}' on #{name} accessory"), verbosity: :debug
           puts_by_host host, capture_with_info(*accessory.execute_in_new_container(cmd))
         end

lib/kamal/cli/app.rb

@@ -127,6 +127,7 @@ class Kamal::Cli::App < Kamal::Cli::Base
       say "Get most recent version available as an image...", :magenta unless options[:version]
       using_version(version_or_latest) do |version|
         say "Launching interactive command with version #{version} via SSH from new container on #{KAMAL.primary_host}...", :magenta
+        on(KAMAL.primary_host) { execute *KAMAL.registry.login }
         run_locally do
           exec KAMAL.app(role: KAMAL.primary_role, host: KAMAL.primary_host).execute_in_new_container_over_ssh(cmd, env: env)
         end
@@ -152,6 +153,8 @@ class Kamal::Cli::App < Kamal::Cli::Base
       using_version(version_or_latest) do |version|
         say "Launching command with version #{version} from new container...", :magenta
         on(KAMAL.app_hosts) do |host|
+          execute *KAMAL.registry.login
+
           roles = KAMAL.roles_on(host)
 
           roles.each do |role|