bootstrap_container_definition should be merged with defaults

Let's see if this idea and solution stands before test writing.

Is there ever a reason to unset any key from the container definition? If so then that is not possible with merging.

I do not think there is a use case for unsetting, only overriding.

Let's say we want to swap out cache.container for a different backend. We probably want to replace the arguments array - but NestedArray::mergeDeep() will append new arguments instead of replacing:

$bootstrap['services']['cache.container'] = [
  'class' => 'Drupal\Core\Cache\DatabaseBackend',
  'arguments' => ['@database', '@cache_tags_provider.container', 'container', Drupal\Core\Cache\DatabaseBackend::MAXIMUM_NONE],
];

$bootstrap2['services']['cache.container'] = [
  'class' => 'Other\Service',
  'arguments' => ['@other_backend'],
];

> Drupal\Component\Utility\NestedArray::mergeDeep($bootstrap, $bootstrap2);
= [
    "services" => [
      "cache.container" => [
        "class" => "Other\Service",
        "arguments" => [
          "@database",
          "@cache_tags_provider.container",
          "container",
          -1,
          "@other_backend",
        ],
      ],
    ],
  ]

Believe NW is the current status but let me know if I'm wrong.