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

test/cli/accessory_test.rb

@@ -115,6 +115,7 @@ class CliAccessoryTest < CliTestCase
 
   test "exec" do
     run_command("exec", "mysql", "mysql -v").tap do |output|
+      assert_match "docker login private.registry -u [REDACTED] -p [REDACTED]", output
       assert_match "Launching command from new container", output
       assert_match "mysql -v", output
     end