Skip existing containers on accessory boot

When booting an accessory, check for the container first and skip boot
if it exists. This allows us to rerun `kamal setup` on hosts with
accessories without raising an error.

Fixes: https://github.com/basecamp/kamal/issues/488

test/integration/accessory_test.rb

@@ -17,6 +17,9 @@ class AccessoryTest < IntegrationTest
     logs = kamal :accessory, :logs, :busybox, capture: true
     assert_match /Starting busybox.../, logs
 
+    boot = kamal :accessory, :boot, :busybox, capture: true
+    assert_match /Skipping booting `busybox` on vm1, vm2, a container already exists/, boot
+
     kamal :accessory, :remove, :busybox, "-y"
     assert_accessory_not_running :busybox
   end